How many bullets does a machine gun hold?

Belt Fed - Average is 250 rounds, although more can be linked together to form a longer belt.

Mag Fed - Average is 50 rounds (FNFAL LSW and BAR) not too many mag fed LSW's out there though.

The canvas belts in the maxim and vicars MG's held:

Vickers - 300 rnds

Maxim - 150 rnds (reduced to 100 in Russian service)