I try to maintain a good quantity of loaded ammo that I shoot alot of. For me thats 9mm and 223. That means a couple thousand of each, built on proven tested handloads, that function well in all my guns. I have 3 9mm's and 2 ARs, so I base my needs off of how many of each I might want to shoot.
Then I have calibers that I shoot that I dont keep alot on hand for. 45, 308, 300 BO. I have thousands of 45 brass, but I only one a single 45, so I keep 500 rounds on hand. The same can be said for my 308 and 300 BO, I keep a few hundred of each loaded in case I feel like shooting, but I only have 1000 pieces of brass on hand for each of these cartridges.
I do own a progressive (LnL) so I can load more at will, and quickly. I keep 10k+ primers on hand. I keep about 50lbs of powder on hand. And I have about 20k pieces of brass between the calibers I shoot. In short, I can load more at will, but I keep a good supply of my most common calibers on hand and ready to shoot.