I'm new at this, but . . .
As a general guideline I plan in the direction of 1,000 rounds per weapon, with some considerations, thus:
I have multiple .22 rifles and pistols. Happily, .22 is really cheap, so 1,000 is easy. I also use them for practice and teaching wife/kids, so I keep an additional 1,000 for that as a "practice float." Now I'm working on a stock of higher performance stuff (Velocitor, Yellow Jacket, Viper, and like that), and that stuff is 2x, 3x, 4x as expensive.
Multiple 9mm (rifles & pistols). Again, the goal is 1,000 per weapon, plus some for practice. Given that 9mm costs about 5x what .22 does (at least in FMJ), and given that I do burn a certain amount practicing, it takes more time to build toward that. Along with FMJ, one needs some JHP and the like, and that costs more, but I add about a box of that for every 4 boxes of FMJ.
The .40 profile is like the 9mm, but there's only one of him, so the build-up is a little slower. Less practice with this one, so less burn.
Moving up in price, we have .30 Carbine. Build-up is even slower here, but I don't practice as much with it, so burn rate is lower.
At the top, there's 30-30, and that builds slower than all the rest. That puppy doesn't go to the range every month, cuz until I get a recoil pad on it, my shoulder doesn't enjoy the experience as much as the rest of me. I've got a ways to go to the magic 1,000 mark -- especially since I insist on things like the new Hornady stuff.
When I'm all done, I expect to have about 1,000 per weapon, plus enough practice ammo that I can hit the range with any of the stable and not run myself short.
Once the basic levels are achieved, I begin to replace FMJ with whatever's appropriate for the expected application (JHP, softpoint, and so on) so that, in the end, the reserve stock for any given weapon is what's most appropriate for its most likely usage, plus practice stuff.
Yeah, I wish I had the cash flow to just plunk down and git 'er done. I'm just a working stiff, though, and it takes as long as it takes.
And you will have noticed that not all the calibres are represented. So there will probably be .223, .357, and .45 in my future. And, of course, all the associated costs of building their reserves.
Oh, darn.