I was buying for 2 -- my older daughter and myself.
I had the minimum of 5 types I started buying: shotgun, bolt rifle, semi rifle, revolver, and semi pistol. This was our training mix. Get to know these, and everything else is a variant.
Well, then I wanted a 9MM for her and a .45 for me. A .38 special for her, and a .357 for me. A 10/22 for her, and a 5.56 for me.
We hit 10 firearms very quickly.
I was fascinated with the history of Mosin-Nagants. Started with one 91/30, then added 2 M44s, and a 91/30 Hex. Each has it's own unique characteristics.
Then I wanted to BUILD an AR-15. Then my daughter did, too. I tried a KSG shotgun at the fun shoot, and had to have one.
Then OGC had an Arsenal AK-47 on sale. Then Sig offered 2 free mags, a range pistol bag, and I found the 1911 I wanted on sale all at once!
I gave my daughter 4 firearms to take with her to the mainland when she moved, so that opened up more safe room!
I have 4 firearms I see as collectibles, so I don't intend to shoot them. The rest all have uses or designs that make then unlike the others I own.
I tried to confine my purchases to a limited number of ammo calibers as well: .22LR, 9MM, .45ACP, .38SP/.357Mag, .223/5.56, 7.62x39, 7.62x54R, and 12Ga. I have barrels to swap on one CZ rifle that use .17HMR and .22WM, but I have the .22LR barrel installed now.
That's 3 handgun, 3 rifle, one shotgun, and one dual-purpose ammo (.22LR for handguns and rifles).
I'm very content with my collection. What I do now is decide on what my next one will be IF I find a very good deal on it. I ignore all other "great buys" until I take care of the one on my list. Saves me from a constant cycle of impulse buys and needing another safe!