In a medieval feudal system, how many knights and warriors did a vassal have to give to the king in time of war?
This would have been different in each individual case. The lord grants lands and protection to the vassal to exchange for obligations and fealty. Just what a vassal got differed from case to case -- the right to the taxes from a mill, a couple of villages, an entire county, whatever. And what the vassal owed was usually roughly in proportion to the fief.
This goes all the way up and down the chain. The serfs of one village might owe their lord different things than the serfs in another village, just because an ancient contract spelled it out that way.