It also depends with me. For each of my hunting rifles I keep at least 3 or 4 20-round boxes of loaded ammo for each weight bullet and for each rifle. In addition, I keep at least another 2 or 3 50-round boxes of parctice ammo for each rifle.
For my shotguns, I keep a flat (10 boxes) of loaded target ammo for each guage (12, 20, 28, and .410), plus a flat each of Trap Singles and Handicap shells. That's way down from when I was competing in either Trap or Skeet where a weekend shoot where just to shoot each of the events you would need 400 to 700 shells. I remember one Trap shoot where the event one day was 500 singles targets. At another Trap shoot 5 of us tied with 100 straights in the 16 yard singles, and I went straight in the shoot off until I misses my 97th bird.
I don't compete in any pistol events, but I reload 6 different pistol cartridges and I keep at least 6 50-round boxes for each cartridge, plus additonal boxes for each cartridge with different weight or style bullets. For the pistols (and my .223) that I shoot almost every week, I also keep a quart jar bulk loaded with reloads for each of those pistols and my ,223.