The Soviet fighters armed with 20mm cannons usually carried from 100 to 200 rounds per gun. The 20mm ShVAK cannon had a rate of fire of about 12 rounds per second, so this provided about 8 to 16 seconds of firing. This was typical for cannon-armed fighters of WWII.

For example,

Fighter	Gun	rounds/gun	firing time
Lavochkin La-5	20mm ShVAK	170	14
Lavochkin La-7	20mm ShVAK	200	16
Lavochkin La-7	20mm Berezin B-20	100	8
Lavochkin La-9	23mm Nudelman-Suranov NS-23	75	8
Polikarpov I-16	20mm ShVAK	150	12
Yakovlev Yak-9	20mm ShVAK	120	10
Messerschmitt Bf 109E-3	20mm	60	7
Messerschmitt Me 262A	30mm	100	10
Mitsubishi A6M2 Zero	20mm	60	7
Mitsubishi A6M5b Zero	20mm	125	15
Supermarine Spitfire Mk.XIVE	20mm Hispano	120	12
Hawker Tempest Mk.V	20mm Hispano	200	16

Some of these aircraft also carried machine guns in addition to cannons, and some other aircraft were armed with machine guns only. 200-300 rounds per gun was common for machine guns, usually providing 15 to 30 seconds of fire. The Messerschmitt Bf 109E-3 was exceptional for a single-engine fighter in that it carried 1000 rounds per machine gun, for 55 seconds of fire. Some two-engined fighters also carried more rounds per gun.

To summarise, for cannons, ammunition per gun providing 7 to 16 seconds of fire was usual, and about 15-30 seconds for machine guns. Soviet fighters carried ammunition per gun comparable to the fighters of many other nations. Some of the Soviet fighters were fairly small, and carried fewer guns than larger fighters, but they usually carried a similar number of rounds per gun.