Ammo available. How much should i own?
Firearm ownership is a responsibility. There's skill involved. Anything that involves skill requires practice. Each weapon is a little different, reloads to the point of muscle memory. I like to have at least 1000 rounds through each weapon to ensure that I am a) completely comfortable with it's operation and b) confident in it's reliability and that it won't let me down when it counts. And if it does jam, fail to extract, load, etc I can react in quick fashion.
I have 5k rounds through my ps90, it's my favorite home defense gun due to it's extremely small profile in an sbr form, minimimal recoil, lethality of the round, and magazine capacity. It has never had a malfunction.
My ccw that I had for nearly 20 years had about 10k rounds through it. It was the Ruger P89, and I still have it, but it's no longer my ccw. I can count the malfunctions on one hand, and I suspect it was due to ammo quality, and not a fault of the weapon.
My primary battle rifle I have about 3k rounds through, it's an AR. But I have many AR's, and I like to keep them in regular circulation to keep round counts from going through the roof.
I have numerous subguns in 9mm, scorpion, stribog, ap5, couple ak's, that are lighter and less recoil than AR's that the children can handle when they're looking to shoot something more substantial than a handgun.
So to answer your question, there's no one answer fits all. It depends on what you use it for. Did you buy a 9mm handgun and stash it in a fingerprint safe incase something goes bump in the night, 10 years ago, and nothing has gone bump in the night, so it remains there, undisturbed, and you hope that you'll remember exactly what to do years later if something does go bump in the night? Or are you shooting regularly, to retain your proficiency level?
If you've just got a 9mm and a 12ga, and shoot occasionally, I would say 6 boxes of each if you're not worried about anything coming down the pipes. If you shoot every week, have a few dozen rifles, I would say more. Ultimately, it is what kind of shooting you do. Hell, I have 5k of .22 for varmint's alone.