For me, it depends on the gun and its intended use.
.30-06, mostly M2 Ball-equivelant for use in the Springfields&Garand, and for that reason I try to keep 4-500 rounds on hand, in clips and bandoliers. I reload pretty much only to replenish what I shoot at a range session, usually 40-50 rounds.
For hunting rifles I keep a couple boxes of pet loads on hand ready to go, and they linger a long time as I just don't hunt as much as I used to. If trying a new load for a new rifle, I load just enough to accomplish finding a pet load and then I follow the same protocol for it as I do the previous hunting rifles. These guns are .30-06's (aside from the above mentioned SpringfieldsGarand which serve a different purpose), and .223, .250-3000, 6.5x55, and a couple obscure calibers (all single shots) and a couple Hornets.
Most of my shooting though is with cast bullets anymore, and if a plinking session is in store for any of the above rifles I'll load up maybe 100, when I need them. The target guns (all traditional under .40 calibers), and all single shots, pretty much don't get any ammo loaded for them at all (but a helluva lot of bullets are cast) as they're loaded at the shooting bench with bullets seated separately into the breech ahead of the charged case which gets used over and over virtually forever.
Pistols, a different scenario all together. Usually load them in batches of 100 and try to maintain an inventory of around 500 rounds of pet loads for each caliber, again exclusively with cast bullets.
