I'm reading Thackeray's Vanity Fair, and several chapters are devoted to military spouses traveling to Belgium alongside the English regiment during the end of the Napoleonic Wars. How common was this in practice in the English military? For what ranks of soldier was it considered acceptable? When did the practice begin and when did it end? It seems like it would have introduced a lot of logistical complications.
Most of my knowledge is about the Peninsular Campaign, so that is what the majority of this is based upon.
Army wives were very common throughout the Napoleonic wars. In 1800 the Duke of York issued a proclamation to the British Army that for every 100 men 6 wives were allowed to accompany them. These women would receive half rations and any children (women with children already were usually not allowed to embark so these would be children born on campaign) would receive a quarter ration. The women were generally chosen by lot. As far as I am aware there was no quota for officers, whose wives could accompany them if they wished. These women would offer a great deal of service in camp, washing and sowing clothes to earn money, cooking and often attending the wounded. Both Wellington and Napoleon were personally against these orders as they felt that women were both a hindrance and, according to Wellington “It is well known that in all armies the women are at least as bad, if not worse, than the men as plunderers, and the exception of the ladies from punishment would have encouraged plunder".
Those women who followed the army would have to face the same hardships as the men serving, and I recommend you look into the Retreat to Coruna in 1808 to read some true horror stories (one that always stayed with me was a couple who lost both of there children in the chaos and had to embark on ships back to England without them, as well as the story of a lady who had to give birth in the snow by the side of the road as the army retreated)
The biggest contingent of women following the army however were not those who came from England, but those who joined the army once in Portugal and Spain. There was no real way to count the numbers, but I have read several accounts of thousands of women being abandoned in Portugal at the end of the campaign.
The most famous woman to join the wives of the British Army was probably Juanita Smith, who after being orphaned at the Siege of Badajoz fell in with and married Sir Harry Smith of the 95th Rifles. She was 14 at the time (which certainly raises eyebrows today) and they remained married for the rest of their lives and she accompanied him all over the world.
The general practice of army wives appears to have died out during the middle of the 19th century as the professionalism of the armed forces improved, and the services rendered by these women were carried out by the army itself.