I think we should distinguish between training/play ammo and "serious" ammo (i.e. stuff intended for hunting, self-defence, etc.).
In the latter category, I subscribe to the "200-round reliability test" theory - namely, that in order to fully trust your defensive weapon and load, you need to put 200 rounds of
that ammo, in
those magazines, through
that weapon, without a single malfunction or problem of any kind, before you can fully trust it. (Of course, this applies to autopistols, not to revolvers, which don't have the same feeding issues.) So, I'll find a load that works well "on the street", and is generally considered a good defensive round, and then I'll try to buy at least 1,000 rounds of it. Some of that will go towards the reliability test: since I typically have duplicates of my carry weapons (in case one has to go for ballistics testing post-defensive use, or needs gunsmithing), each gun will get 200 rounds for testing. After satisfactory completion of the reliability tests, I usually try to keep 300-500 rounds of that load on hand, allowing for rotation of carry ammo on a regular basis. If I have to buy more, I check to ensure that the bullet shape, dimensions, etc. are still the same (it's surprising how these can change sometimes, without the manufacturer bothering to let you know!). If they're identical, I don't worry about repeating the reliability test, but if there's any change, I have to start the testing process all over again.
In hunting ammo, I try to find a load that works well on the animals and in the field conditions concerned, and then keep at least 100 rounds of it on hand, for annual re-zeroing of my rifles, and for hunting use. 100 rounds will typically be enough for 4-5 years of hunting.
For practice ammo, well, I try to stock up at the beginning of each year, as I can buy in bulk for rather less than normally-advertised prices (special deal through a local LE department). I may have as much as 10,000 to 15,000 rounds on hand early in the year, in my carry calibers, and go through that during the year in courses, normal range work, etc. I try not to drop below 1,000 rounds in my most-used calibers, and not below 500 rounds in my less-used calibers.
Here's what my ammo locker looked like at the beginning of 2004, stocked up for the year and not yet depleted.