Per Table of Organization and Equipment 7-17 (Infantry Rifle Company), dated 26 February 1944, the U.S. Army rifle squad was, on paper, composed of the following;

Men:

Quantity	SSN	Rank	Role
1r	745	Staff Sergeant	Squad leader
1gr	745	Sergeant	Squad leader, assistant
7r(2gr)	745	Pvt./Pfc.	Rifleman
1b	746	Pvt./Pfc.	Rifleman, automatic
1r	746	Pvt./Pfc.	Rifleman, automatic, assistant
1r	746	Pvt./Pfc.	Ammunition bearer

Equipment:

Equipment	Quantity
Launcher, grenade, M7	3
Rifle, Browning, automatic, cal. .30, M1918A2	1
Rifle, U.S., cal. .30, M1	11

• b: Armed with rifle, automatic, cal. .30

• g: Armed with launcher, grenade, M7

• r: Armed with rifle, cal. .30, M1

Each man in the squad, save the automatic rifleman, would have an M1923 cartridge belt capable of holding ten en-bloc clips (80 rounds) for the M1 Garand rifle. Additional disposable cotton bandoliers each holding six en-bloc clips (48 rounds) could be obtained. Each member of the squad was issued with a general purpose ammunition carrying bag, while the ammunition bearer in the automatic rifle team was issued with two. The bag could hold, among other things;

• 4 M1 rifle ammunition bandoliers

• 14 30-round M1 or M3 submachine gun magazines

• 10 BAR magazines

• A 250-round belt of .30 caliber machine gun ammunition with or without can

• 5 rifle grenades in fiberboard packing tubes, or 11 without

• 11 hand grenades in fiberboard packing tubes, or 28 without

• 11 hand grenades attached to M1 grenade projection adapters

• 2 60 mm mortar rounds

• 4 M7 mines

The allocation was increased to 20 bags per squad (the seven riflemen being given one additional bag) on January 30, 1945 with change 2 to TO&E 7-17.

The Browning Automatic Rifle gunner was equipped with an M1937 cartridge belt, capable of holding up to twelve magazines for the gun (240 rounds), although many gunners carried less than that due to the sheer weight of a loaded belt. The assistant gunner and ammunition bearer were also initially issued M1937 belts along with their M1923 cartridge belts, with the expectation being that the M1923 would be used for garrison use, while the M1937 would be used in the field; they would carry rifle ammunition in their general purpose ammunition carrying bags. On June 30, 1944, Change 1 to T/O&E 7-17 reduced the number of M1937 cartridge belts in the rifle squad from three to one; the assistant automatic rifleman and ammunition bearer retained their M1923 belts.

Men issued with M1 carbines received a varying number of ammunition pouches to place on their M1936 cartridge belts depending upon branch of service;

Number	Branch
1 (30 rounds)	Army Air Corps, Medical Department, Ordnance Department, Transportation Corps, Quartermaster Corps
2 (60 rounds)	Adjutant General's Department, Antiaircraft Command, Chemical Warfare Service, Coast Artillery Corps, Corps of Engineers, Field Artillery, Infantry (including armored infantry), Military Police Corps, Military Railway Service, Signal Corps, Tank Destroyer Force
3 (90 rounds)	Armored Force
4 (120 rounds)	Parachute and glider infantry
5 (150 rounds)	Cavalry (mechanized or horse-mounted)

Change 2 to TO&E 7-17 replaced the carbine-specific ammunition pouches with pouches that looked very similar, but whose compartments were redesigned to be spacious enough to hold an en-bloc clip for the M1 rifle; the pouches had previously been issued to airborne and mountain troops.

Sources:

Rottman, Gordon L. World War II U.S. Army Combat Equipments. Oxford: Osprey Publishing, 2016.

United States. War Department. War Department Basic Field Manual 23-30 Hand and Rifle Grenades, Rocket, AT, HE, 2.36-Inch. Washington: United States Government Printing Office, 1944.

Although u/the_howling_cow has already answered the US part of the equation, I can give a quick overview of WWI ammo count, based on the excellent book "Field equipment of the European Foot Soldier 1900-1914", which was originally published in French by Commandant Emile Charles Lavisse, and was updated and translated later on.

The book gives the following table of ammunition carried in this period, and it should be quite reflective of WWI:

Country	Cartridge	Amount
Italy	6.5×52mm Carcano	162
Spain	7×57mm Mauser	150
Germany	7.92×57mm Mauser	120
France	8×50mmR Lebel	120
Belgium	7.63×53mm	120
Norway	6.5×55mm	120
The Netherlands	6.5×53.5mm	120
Russia	7.62×54mmR	120
Switzerland	7.5×55mm	120
England	.303 British	100
Austria	8×50mmR Mannlicher	100
Denmark	8×58mmR Krag	100
United States	.30-06	100
Sweden	6.5×55mm	100

Note that this is an on-paper distribution of ammunition. During wartime, it's very common for soldiers to carry extra ammunition, be it in a backpack, breadbag, uniform pockets, or bandolier.

As for the various nations during WWII, it's a little more complicated: At this stage, light machine guns had become common, and most riflemen acted as ammunition bearers for these weapons as well.

The German infantryman carried 60 rounds of 7.92×57mm in his Patronentasche 11, and was also responsible for carrying part of the 2500 rounds allotted to the MG. With the MG crew carrying a substantial part of this, the German infantryman would carry perhaps 200 to 250 rounds for the MG, depending entirely on the location, squad size, situation, and time during the war.

The Dutch Infantryman carried 60 rounds of 6.5×53.5mm in a rifle pouch, and a single Lewis drum of 97-rounds in a round pouch. If his squad had no machine gun he carried 120 rounds of rifle ammunition.

The Belgian infantryman carried 120× rounds of 7.65mm Belgian for rifleman, with another 720 rounds of ammo spread amongst the 6-man BAR crew.

For the British section (which simply means squad), each man of the 1939 section carried 50 rounds of .303 per man, and 3× 30-round Bren magazines.

The 1942 section carried 50 rounds per man, with a total of 22 magazines spread amongst 8 men. The squad leader carried a Tommygun, with 6 magazines of 20 rounds.

The French infantryman, I'll simply leave this image here.

The Polish squad carried 60 rounds in pouches plus 30 in reserve, whilst its 4-man BAR team carried 500 rounds with another 540 in reserve on the ammo wagon.

I could go on and on, but the basic idea remains the same: Before WWI 100 rounds plus was carried, whilst after, you're more likely looking at 60 rounds plus light machine gun magazines. Please note that these are just "on paper" amount of ammunition. The amount of ammunition carried will change according to the situation, location, task, time during the war, etcetera.

Sources

WWI: Field equipment of the European Foot Soldier 1900-1914

German: http://www.lexikon-der-wehrmacht.de/Soldat/Bekleidung_Ausrustung.htm

Dutch: Thalens, Ransel op de Rug, plus Handboek voor den Soldaat 1938

Belgian: Uitrusting van het Voetvolk, 9 Jan 1940

British: 1939 and 1942 Bren Manuals

French: David Lehman, see image

Polish: http://wp39.struktury.net/pulk-piechoty-pluton-strzelecki-1939.html