The answer, as always depends on what you are stocking up for. If you worry about TEOTWAWKI then you need all the ammo you can get. If you worry about less pressing matters then you would reduce the supply to a couple years of your estimated use.
I have 3 storage levels;
Deep storage (long term storage for when everything else is gone)
Ready storage (mid term storage for when the S hit's the F)
Using ammo (what you use regularly)
My rule of thumb is;
Deep storage;
500 rounds for each handgun, 1000 rounds for each rifle, 250 shotgun shells of each TYPE (250 00 buck, 250 71/2 shot, 250 slugs etc.) for each weapon and 10,000 rounds of .22lr. All crated for long term storage (sealed tins are best). Note I say for each WEAPON, that means if you have 4 9mm's you should have 500 rounds for each. I would suggest that this storage be kept for only your SHTF weapons. Pick the ones you plan to use ONLY for survival and not every weapon you own. This stuff should be stored to maximize storage life even if that means it is not readily accessible, it is meant for when everything else has run out.
Ready storage;
This should be about the same as the deep storage but stored where it can be gotten too. I suggest using ammo cans for this as tins can be a bummer to get open in a hurry. This is your go-to ammo when you normal supply is running out and you need to use it without wasting a half hour getting at it. You can cut down on the amount of .22lr and shotgun ammo if you want but keep the centerfire rifle and pistol high as this is the stuff you fight with.
Using ammo;
What you shoot all the time and including all the various calibers and stuff. Figure out what you shoot in a year or two and double it for safety. You reloading stuff should be in this category as well unless you can insure that you can store the componants in a manner that keeps them useful for the long term.
The key to keeping this to manageable levels is to define what you need for various tasks. Pick the best weapons for the purpose you envision, keep them the same (all Glocks, 1911's, whatever) and minimize the calibers to reduce volume. If you like .45acp and your life-partner prefers a 9mm pick the one you can both handle and use only it for SHTF use, or pick a compromise like the .40 S&W, whatever. Remember, the guns for TSHTF are for just that, if you want to pack your .45 or .44mag in the meantime cool, but get a dedicated SHTF weapons as well. You should also stick to common calibers, .357 SIG and 10mm are good rounds but 9mm, .40 S&W and .45acp are more available and you are more likely to obtain them easier through barter or gleaning than the oddballs, being able to obtain ammo through other sources will keep you from having to use your stored stuff. If you decide to go with a less common caliber, like the 5.45x39 or 7.62x54r because you can get it cheap, then at least double the quantities for each level of storage.
Remember too, keep large quantities of accessories stored too. I know a guy with 5,000 rounds for his AK and 5 magazines he also has no spare parts and only a small quantity of cleaning supplies, he is NOT prepared IMO. A magazine fed weapon without magazines is a crappy single-shot, and a broken part makes it a club and a weapon you cannot maintain will soon be useless. If you have 3 magazines for your EDC pistol you should have at least as many spares in each level, rifle mags should be kept in bulk, I want at least 25 and 50 or more if possible. One other thing is training, if you use a scope, that's fine but make sure you have good iron-sight as backup. STOP DUMPING YOUR MAGS ON THE GROUND! Most schools teach you to drop your mags for faster reloads but in a SHTF situation any mag you drop is likely to be lost or damaged, the SEAL's and Marines can afford to replace them, can you?