Well, this is a very subjective topic. It's depends on a lot of various aspects. First, what sort of shooting you do is a major factor. For example, I used to do a lot of competive shooting and tactical classes. In a IDPA shoot, it's pretty normal for me to use 100 or more rounds of ammo, for a 3 day tactical class, easy 500 and possibly a lot more. I was shooting about 4 competitions a month and 2-3 classes per year.
So 400 rounds per month in competition of say, 9mm, plus 2 classes per year in let's just say 9mm there too is 5800 rounds per year.
I'm am WAY above normal, and shooting competitions much less frequently now, but even so, I keep ammo around for that. So my numbers are great sku'd.
If you add in "prep" for the zombie apocalypse well...
Right now an election is coming and the economy is weird. So...
I think conservatively for monthly range trips (50-100 rounds ??) in a single (the one you shoot most frequently) pistol caliber, I'd say 1000 rounds of that ammo is what I would suggest. That's a years worth of range ammo with a smaller amount of "zombie emergency ammo". (Zombie ammo needs fall into the super Grey area of prep world and I'm certain there are folks with millions of rounds)
Personally, at that level, I'd refresh monthly for a month. Basically, get a store of around 1000 then buy around 100 per month and the ammo bills won't ever get too big, particularly with an election on the horizon. Shit ALWAYS goes wonky in these years.
Quick side note: I'm super finicky when it comes to "Zombie ammo" and a lot less so on "practice ammo". I set up notifications when multiple specific ammo becomes available for both. Check prices when they do, and if they fall below x threshold pic up a few boxes. I'm like a fucking day trader with ammo and during covid sold enough ammo to my LGS (who's also a friend) to pay for a CCMG AR in 9mm.