The answer is another question: What kind of SHTF?
Deep economic SHTF, as in: Life goes on, but your ability to procure more, due to your finances is limited, or due to the market even if you're one of the few with money?
The answer, "As much as you can."
Political SHTF, as in: Your ability to buy more ammo has been legislated away? (IMO, time to use that ammo, but that's another thread... so I digress.) The answer, "As much as you can."
A localized/regional SHTF, such as an earthquake, a tornado, wildfire, or a Katrina where there's also a looting/social unrest? The answer to that is "A single combat load in your chosen handgun and long-gun caliber that you don't dip into during your normal range time." Because if you need more than that, unless you're part of a larger group, you and your family are likely toast.
Total Mad-Max TEOTAWAKI SHTF, with physical, economic, and .gov disaster, that is both pervasive and prolonged? I think it matters if you can "bug in" or if you must "bug out". Bug in, the answer is kind of unsatisfactory. "As much as you can", but my gut reaction is, if you need more than a single combat load, if you're not part of a group, you're probably done for. Bug out, it's what you can reasonably carry, and still carry all your other necessities. Which pretty much means "A combat load" again.
The common caliber advice, 7.62x39, .223/5.56, 12ga, .22LR, .30-06, 9mm, .45acp etc. is sound. But if you need more, post-SHTF, you're going to be bartering for it, and it means you've got stuff people really NEED. Food, medication, fuel, etc. Because ammo is a close second, because it's your ability to actually defend and hang onto those necessities. If you've got the stuff to barter for ammo, then it sounds like you're already a well set up individual or family to me already. So why didn't you have enough ammo in the first place?
Police stations? If it's "that bad" the police will have already distributed or actually used the ammo. If whatever it is happens so fast that the police don't get to their own supplies in the station, then odds are things are so dangerous, flood, fire, fallout, riots... that you probably have no business going after it yourself. And even if the ammo is loot-able, someone with fewer social scruples than you will have gotten to it first.
I hear NG armories actually rarely, if ever, have live ammo. But if they did, the case would hold as it does above for the police station.
So the real possibilities are "battlefield pickups" from what you find or from those you yourself kill. So in that regard, it comes back down to the fact that unless you're part of a superior group, is wherever there's that many dead, or that many you need to kill, is this a place you should be?