There is no "standard" for extra ammo, per say...at least not Army wide....when I was in Iraq though we loaded every vehicle with as much ammo as it could carry of any type we could get our hands on. My 1114 contained an empty Mk-19 ammo can that we had our engineers bolt to the turret filled with extra grenades and other pyro that the gunner could dig into if we needed it, a few AT-4's strapped to the back of the turret, in the crew compartment we had 6 cans on 7.62mm link (or 5 cans .50 cal API and 1 can .50 cal SLAP, obviously depending on the weapon system), a pyro bag filled with grenades, smoke, flares, etc., and an ammo box filled with spare prefilled M-16 magazines, and in the trunk we had 2 cases of 5.56mm ball, and 1 case of 5.56mm link.
Unfortunately we couldn't get rockets for it, but I wanted to toss in a SMAW from the Marine unit we were attached to. That would have been nice.
Anyway, yea, assuming that your higher unit can get it, spare ammo isn't really a problem when you have vehicles, whether it's a HMMWV, Styker, Bradley, whatever.