It's a silly question. "Enough to do WHAT". Enough because they stopped manufacturing ammo and you want enough to continue plinking every weekend until you do 40 years from now??? Enough in case a bad guy(s) break into your house and you have to protect your house and family????
The truth is, if you're talking for self defense, then the answer is 1 box of quality SD/HD ammunition. On top of that, should be however much ammo you are going to practice with. Assuming for a second that there is no ammunition shortage; people's conspiracy theories about the government restricting ammunition don't exist; and any other factor that could lead to hording. In a normal situation, all you need is 1 box of good self defense ammo for each gun you plan on using for self defense. Then, buy what you need to practice with as you use it. Preferably you won't be foolish enough to be practicing hundreds of rounds of expensive SD/HD ammunition.
As for plinking, have enough for each gun so that you can go plinking at a moment's notice without having to go buy some. Then replenish the amount after you are done plinking. So, if you normally plink with a box each of 45acp, 357mag, and 9mm; then have a box of each on hand so you can go when you want to. If you go plinking on a regular basis; e.g. every weekend; then you could have enough plinking ammo on hand for 2-3 trips. Then restock as convenient.
Now; for the $1,000,000 question. If you're talking about "Enough Ammo per gun" in case of conspiracy issues where uncle SAM is going to take away the ammo or make it difficult to buy and thus their way of getting around the 2nd amendment. "Can't get rid of guns, tax the hell out of ammo or make it impossible to buy". Well in that case, there is only a few possibilities.
1) If you want to still be able to go plinking, at the same level you are now, then you need to add up what your using in a month and multiply that out for the next however many years you plan on being alive.
2) If ammunition is going to somehow become illegal, monitored, serial numbered, etc... then plinking will pretty much become a thing of the past. Because as you're seen or notice plinking, "THEY" will be coming after your stash because it's not "Legal" stash. Obviously, this is a little unlikely. They would simply make all "NEW" ammo registered and such. They'll let existing ammo go out through attrition. However, just like anything else, prohibition doesn't work. There would be a black market.
3) The only other scenario would be if ammunition was controlled in such a way that you no longer could go plinking. In that case, you're only stocking for SD/HD use. However; the first time you need to use it for SD/HD use, the authorities will know, and you'll lose your stash.
So, there is no answer that is valid. The best case scenario is that ammo will still be legal, just that the government will want to make it difficult and too expensive for the average person to have. Which isn't logical, because all that would do is stop PLINKING. Plinking isn't where people get shot. So everyone would still have a couple of boxes for SD/HD. But for those wanting to have enough to continue plinking in a manner that they are use to, then the answer is simply to figure out how much you plink now, and store up enough for the next 40 years or so. If it's for SD/HD only, then you don't need but a couple of boxes. "No, this isn't a Hollywood movie where you're going to need to shoot up hundreds of people". And if that scenario did exist, then a gun is just one of many tools you better have. Those include other self defense methods, survival skills that don't include a gun, and other skills. Guns are JUST A TOOL. They aren't the answer to your survival. YOU are the answer to your survival. Try fixing a car with JUST 1 SCREWDRIVER!!!!!!! Well, believing that a gun is all you need for survival is the same thing. You CAN'T survive if you think you only need a gun and enough ammo.