It depends on the usage for me.

I don't shoot my 300 Savage or 308 much, so I usually only keep 50-60 loaded rounds for each.

Now, something like 32 S&W Long, 45 Colt, or 303 Savage that I take out and plink a bunch of ammo through for fun, I usually keep 200-500 loaded, because I might go out and blow through a couple hundred rounds on a longer plinking day.