How extensive was the use of white mercenaries in the post-colonial troubles in Africa?

The use of Europeans as mercenaries in Africa during the post colonial period was a feature of the times. It has been argued that the colonial powers had during the training of native troops instilled in them a sort of lack of self-confidence that they were not the equal to European troops. This made white mercenaries a rather potent fighting force against native troops,as the native troops had been taught they were not as good as the Europeans. The most spectacular or notorious use of mercenaries was during the Katanga Succession.
The ethnic people of the province of Katanga declared independence from the newly de-colonized Republic of Congo. Katanga was rich in mineral wealth, so the newly formed government was not to hot to let the province go. (There are others here who can do the story of the Congo politics in the 1960's better than I can.) Moise Tshombe of Katanga originally had a great deal of Belgian support, but later turned to European mercs to bolster his fighting force. This is where a lot of the legends of white mercs in Africa were born. Folks such as "Mad" Mike Hoare (formed and led the legendary 4 Commando and later 5 Commando in Kataganese service), Jean "Black Jack" Schramme, John Peters, Rolf Steiner, Robert Faulques, Taffy Williams and Robert Denard created the mercenary legend during these years as swashbuckling soldiers of fortune. Their troops were mainly Belgian, English and French, but pretty much all Western European nationalities were represented. (No Americans, Hoare has hinted he had an agreement with the CIA, no Americans in the 4/5 Commando and the CIA would keep out of his hair.) They were very effective in terms of combat against Congolese troops, who called them " Les Affreux" or "The Terrible Ones." So effective at what they did, it looked as if Katanga would be independent of the Congo, keeping all of its mineral wealth to itself (and perhaps to the benefit of its former Belgian owners). The conflict ran roughly from 1961-65, but the UN eventually forcibly repatriated Katanga back to the Congo. This was not the end of the merc in Africa, you would see their return in various other conflicts over the years. One of the more notable instances was in Biafria where the Igbo people attempted to succeed from Nigeria. They had a fairly inexperienced army, but white mercenaries, notably Rolf Steiner and Taffy Williams, added a great deal of fighting power to the Biafria rebels and even created a nascent air force (one pilot was the famed WII ace Jan Zumbach). This helps illustrates why mercenaries saw use in Africa, they provided a level of expertise that native armies simply did not have, such as building a fighting force from the ground up and training it. But Biafra and Congo were the two "main" mercenary wars that brought attention to the white soldier of fortune in Africa. You would see further mercenary use in Angola, Bob Denard would try and knock over the government of the Comoros four times, Mike Hoare would be captured during an attempted coup in Seychelles in 1978. You would also see what some might call mercenaries in the Rhodesian and South African military. Short on trained personnel, the Rhodesia recruited out of work soldiers to fight for them in the 1970's, mainly Portuguese veterans of their own colonial wars. The Rhodesians did have a sizable all French speaking unit for a time, and there are comments from former Rhodies that indicate a goodly number of US veterans from Vietnam fought for Rhodesia as well.

Mike Hoare, The Road to Kalamata

Mike Hoare, Congo Mercenary

Christopher Othen, Katanga 1960-63

Peter McLaughlin, The Rhodesian War: A Military History.

Guy Arnold, Mercenaries: Scourge of the Developing World

I always knew that mercenaries were part of the civil wars and revolutions the plagued post-colonial Africa, but the film implied that they were some of the most important and reliable troops used.

By and large, they were, at least in the Congo. The mercenaries that served in Africa were seasoned troops, veterans of the Second World, Korea, Indochina, and Algeria, amongst other conflict zones. The post-colonial national armies of the newly independent countries, the Congo in this context, were either the nationalized local colonial forces, whose own quality was rather hit-or-miss in the best of times, and only made worse by the utter lack of an experienced, native, officer corp prior to independence. The Belgians only allowed the first class of Africans into the University of Lovanium in Leopoldville in 1954. As higher education was a requirement for officer ranks, even if the Force Publique permitted African officers (it did not,and one was promoted to senior NCO ranks purely by time-served, and rarely by merit), there were simply no qualified Congolese for the senior positions. Couple that with an entrenched colonial mindset in the white officers that ran the Force Publique, the colonial gendarme, and when Africanization was forced upon the Army by Lumumba...disaster was the only reasonable thing that could follow. The mindset of many white officers in the Force Publique can be best epitomized by Émile Janssens' famous blackboard scrawl on 2 July 1960 at an 8pm meeting with senior military officials: "Avant l'indépendance = Après l'indépendance" (Before Independence = After Independence) (SJG Clarke. The Congo Mercenary: A History and Analysis. p 18). which basically told the Congolese army that nothing was going to change. White officers would continue to lead, and the black rank and file would, at best, be senior NCOs so long as the Belgian 'advisers' had anything to say about it. They were a relatively small army for a nation the size of all Western Europe. Because of this mandate the new ANC revolted at Thysville on 5 and 6 July, 1960 (Lawrence Devlin. Chief of Station: Fighting the Cold War in a Hot Zone. p 8.)

The army consisted of only 20,000 men for the whole country at independence, and only 14,000 less than a month later (Clarke, 24). They were poorly equipped, horribly paid, and a mix of the indifference by white officers, a lack of training for a competent officer corps upon the forced Africanization of the Congo, and tribal and personal rivalries and loyalties all weakened the ANC significantly. Thus, for ANYONE to really be effective, they required mercenaries, or at least Western participation, and both sides ended up getting it - Lumumba and later Kasavubu got the UN mission in Congo, which consisted of soldiers from Ethiopia, Ghana, Guinea, India, Ireland, Italy, Malaya, Nigeria, Norway, and other nations with a peak strength of approximately 20,000 (United Nations) in July 1961. Tshombe and Kalonji in South Kasai hired mercenaries instead, especially given that in Katanga, for instance, only 600 of the Force Publique in the province defected. The focus of my answers will be focused on the Katangese mercenaries, especially those that continued to serve and follow Tshombe when he became Congolese prime minister, since South Kasai only hired a few dozen mercenaries, and by and large they were neither as numerous nor nearly as effective as the ones in Katanga.

Now...to touch on the mercenaries: to go further than this we need to separate the two parts of your question: numbers, and then also efficacy/reputation.

As for their numbers, the units were of widely varying sizes. The first round of mercenaries came from Belgium, at the request of Tshombe in Katanga on an 18 September request, numbered 180, to lead and train the Katanga Gendarme (Clarke 24). By February 1961, these Belgian and French mercenaries "numbered 400-500 of whom 210 were Belgian regulars who had worn the uniform of the Force Publique since September 1960" - that is, they were the 180 men plus instructors that Tshombe had asked for on 18 September (Clarke 25). And this number was later boosted to "306 mercenaries, 230 of them Belgian" in country by 14 July 1960, according to Othen (Christopher Othen. Katanga 1960-63.). This is just the number of men that had managed to basically avoid deportation by the UN in the first stages of Operation Rumpunch, organized by Cruise O'Brien in 1961. The deportations included "273 non-Congolese soldiers. Of these, 55 were Belgians, four British, one Pole, one Hungarian, one Dane, two Portuguese, one Swede, eight Italian, one South African, one New Zealander, 11 French and five 'others'" (Tony Geraghty. Soldiers of Fortune. p 43).

In these early days, the Anglo contingent (not just of Britons, but also other Commonwealth states and British colonials, including Rhodesians and South Africans) consisted of about two hundred or so if they were at full nominal strength in the summer of 1961, however they required over 80 replacements by the time Mike Hoare took over this unit, 4 Commando, and based on the numbers from Othen, numbered around 100 men. By 1964, 4 Commando had been disbanded, then reformed as 5 Commando when Tshombe became Prime Minister of the Congo. In its new form, it consisted of approximately 300 men, split into field companies of about 30 men that operated with a great deal of independence, and was nearly at full strength (approximately 400 men) after 270 further replacements were recruited due to attrition from contract non-renewal and the slow but steady drain of casualties (20-30 men killed and wounded on average per month) in the Spring of 1965, which incidentally was the heyday of the mercenary in the Congo. Page 354, footnote 2 of this paper - if you can read French suggests that during this period the number of mercenaries total could be as low as 500, though given the strength of 5 Commando alone at this point, I think that the numbers in Spring 1965 total would range somewhere in the 1,000-1,500 range, with 5 Commando making up probably the single largest contingent under any one commander. Other sources I've seen have listed higher ranges, though few actually agree on the total numbers.

Other mercenary forces led by the other big names - Schramme, Faulques, Denard, et al - led forces of rough similar size, compared to each other, which at their maximal size were of similar size to 5 Commando. The only problem, as it were, is that these men had their time in the sun, largely, during the Katanga Crisis, and many of these leaders had wandered off to other conflict zones by the time Tshombe was given the prime ministry in 1964. During the July 1967 mercenary revolt, for instance, Schramme and Denard each had approximately 400 mercenaries at their joint disposal, and this was basically at their lowest point. At no point when engaged did the European mercenaries make up much more than a hardened core of the military fores present in the various engagements between 1960 and 1965, and their numbers only declined after 1965. As part of a larger force, they were useful, but couldn't do everything on their own, especially against UN forces. They were simply too few to single-handedly carry Katanga to independence.

That wasn't to say that the mercenaries were ineffective against the UN forces, however. It is just to say that they were far from universally successful. "Inexperienced Irish, Swedish, Malay, and Danish soldiers who did not know the neighborhood" were slaughtered and defeated with relative ease by mercenaries that had the support and favor of the locals. Geraghty quotes a mercenary who goes on to say about the operations in Elizabethville: "We knew the town...to borrow a Marxist phrase, we were the fish swimming in a sea of Katangans...crossing the gardens and villas with the complicity of the owners, to throw grenades or hit convoys with bazookas." The fish in the sea comment, of course referencing Maoist theory about guerrilla war. However, their success, even with the first defense of Elizabethville and the costly (though mostly in terms of Katangese Gendarme, not actual mercenary, lives) Katangan victory at Jadotville renewed UN resolve, and the Indian Air Force overwhelmed the mercenaries at the opening of this new offensive in December 1961 as part of the renewed offenseive to put the issue to rest. Despite being beaten, many of the mercenaries were able to largely retreat in good order into Angola. There was a meeting near Kolwezi during this time between Faulques, Schramme, and Tshombe in the aftermath of the decimation on the ground of what passed for the Katangese Air Force (basically 10 T-6 Harvards, 2 Vickers Vampires, and eventually 1 Fouga CM.170 Magister) by Indian Canberra Bombers. It was during this meeting that an exhausted Tshombe was trying to play all sides. He convinced the UN he wanted to surrender. He convinced his loyal secessionists that he would carry on the fight from Angola, and he also promised Faulques and the other mercenaries that, whatever happened, they would be paid in full for their services. Jerry Purren, one of the mercenary commanders promised Tshobme that the mercenaries could hold out in Elisabethville for, at most, ten days. Tshombe promoted Schramme and sent him with his gendarmes into Angola to continue the war from across the border. Denard, always a man with a flair for the dramatic suggested that they "do something...like blowing up the hydroelectric plant at Nzilo." as they retreated. They ultimately didn't do so, instead "extort[ing] 200 million francs from UMHK not to blow the dam" during the retreat after UMHK declared its support for the Leopoldville government. Countless bridges, rail tracks, and the like were blown up however, on Denard's orders during the retreat.