I could keep them all filled, a couple of times over, but I don't. I keep a toolbox that neatly holds 28 30 round mags; those are all loaded. It makes it easier to take them to the range. 10 20 round mags loaded with special purpose ammo (the 69 gr BH and 75 gr Hornady that my precision ARs love) are alos packed up and ready to go. The other 100 or so AR mags are just stored away.
10 M14 mags and 10 FAL mags are loaded and ready to go. Not even sure how many other FAL mags I have around (I know there are 30 1950's FN mags still sealed in their original packing). 25 or so other M14 mag.
Probably 10 AK-47 and 10 AK-74 mags loaded in the range case. I think that at least one of the 100 round chinese drums is loaded, too. I really have no clue how many AK-47 mags I have at the moment (though I know I only have 20 of the Bulgies), and I'm pretty sure there are at least 30 more AK-74 mags (I bought a bunch of the Bulgarian and found the EGs for $4 each in quantity).
Like any good shopper I buy ammo when there are deals and I have the spare cash. It also probably helps that I haven't been able to get to the outdoor range much this year, and shooting centerfire reifles indoors at 25 yards is loud and boring.