I echo the sentiments of the original poster. My circumstances are similar. I have a few “modern” guns and …ahem… a pile, of old relics. These range from 1950s-1700s.
I buy modern .32 ammo when I find it and it’s affordable, to shoot recreationally. I have ~20 boxes at last count. So, plenty for range trips, in one sense. But, not nearly enough at the same time. It’s amazing how shooting a few pistols a couple of times will deplete that stash. (My usual expenditure is about 100 rounds and I have around a half dozen guns in that caliber.)
Of course, there are always guns you find you really like shooting and guns that are cool but you are happy to have shot them once and that’s that. I only make a point to keep the minimum 2 boxes for those.
I try to have at least 2 boxes for each gun, with the idea that I can shoot one if I want and still have one box for emergencies. Even though I have a lot of “collector” pieces, I still feel that a gun is useless without ammo, so I do try to have at least some for each gun. The extra or stockpile is for guns that are actually fun to shoot or make for good range guns. This is tricky for old stuff and means reloading is a necessity.
I do relax my “rule” for some of the black powder calibers though. The fact is, I have a bunch of guns, by the average person’s reckoning. (By gun nut standards I’m probably a solid amateur but I do have plenty.) If I need a gun personally for utility or self defense I’ve got modern ones for the task with plenty of ammo. Ammo for these is also readily obtainable and reasonably priced. But if I NEED that single shot black powder rifle…… My rationale is, if the circumstances are so dire that I truly “need” to use a black powder rifle, then I’m probably using my guns to arm friends and neighbors in some kind of crisis scenario. And if that’s the case, and they actually have to use them, they’ll be at a severe disadvantage, maybe even a fatal one, compared to the guy next to them who has a smokeless powder gun. As such, the appearance of the gun will do enough, and frankly, if things are that bad, they’re probably better off trusting to the guys with smokeless and spending their effort praying, anyway. So I don’t worry overmuch about keeping a healthy stockpile of .577 Snider or .41 Swiss.