KV-1 and KV-1S: 15
M3 Light: 6
The Sherman, despite having similar dimensions to the T-34, can only sit half as many men. That is because there are no surfaces above the tracks to stand on, and not nearly as many handles to hold on to.
Of course, official numbers didn't stop creative arrangement of larger number of infantry. Here you see 8 soldiers riding on a T-70 instead of the "official" 6:
This kind of trick would only work on very good terrain. The front soldier has nothing to hold on to, and, if the tank shakes, too much, will fall off. 8 seems to not be the limit of Soviet ingenuity. According to CAMD RF 38-11355-1420, a T-70 carried 20 men into battle at Kudinovo. The report rightfully complains about insufficient amounts of things to hold on to when carrying that many. Sadly, there is no photo of that, but there is a re-enactment!
The manual also has some data for trucks:
GAZ-AA 1.5 ton truck ("polutorka"): 10-12
ZiS-5 truck: 16-20
ZiS-6 truck: 16-20