.17HMR, .22lr, .223, .22-250, .308, 9mm, .45acp, .38Special, .357Magnum, 20ga, 12ga.
Only one each in .17HMR, .308, .38Special, .357Magnum, and 20ga. Those are "specialty" guns for me, and get the least usage. All the rest have multiple guns. I own more .22-250s than any other centerfire rifle. It does come in handy when I take my sons varminting.
I tend to avoid some specific calibers, especially in handguns. I've moved away from big Magnums (.44Mag +) as I have no large wild animals in my area and don't hunt large game. My love affair with .45acp goes back over 30 years, but I bought my first 9mm last year, as I found myself shooting a lot at indoor ranges, and it's pleasant to shoot while still retaining "real" recoil. Cost of ammo was a huge factor, as well. The .40S&W is a fine round and effectively splits the difference, thus not fitting into my arsenal at all.
Other cartridges I've intentionally avoided: .222Remington, .22WMRF, .25acp, .32auto, FN 5.7,.380auto (owned a PPK/S once, lost to a burglary, never replaced). Had a 10mm slide and mags for my stainless Colt Series 80 - got it used with both 10mm and .45acp setups, sold the 10mm slide/mags/ammo to a local gunsmith, leaving me with a .45acp Series 80 for a ridiculously low net price, so I guess 10mm is also on the list. So far I've avoided .357Sig and .45GAP, as well. Some of these are fine calibers, and if I were starting out today some of my choices might change.
Some of the cartridges I've avoided or ignored have passionate proponents here, and I'm fine with that. More power to them. One of the many great things about this country is we all get to make our own choices! (Some of you guys have more choices available that we do here in California, and I envy you that - I'd have a Barrett .50BMG just to have one, if I could!) It is nice to only have to stock 9 calibers and two shotshell gauges, but if I hit a multi-million dollar jackpot, I could adjust my inventory....
I certainly understand those 9mm/.223 or .223/.40 guys. I don't really get 9mm/.40, but that's their business. I also understand those that try to get as many different ones as they can - especially during ammo shortages. To each their own!