Like others have noted, I'd always start with common calibers and common platforms and make those the core. So for a starter, minimalist collection, to me that would most likely be:
- A Glock 17
- A 12-gauge pump like a Remington 870
- An AR-15, preferably with a .223 Wylde barrel
- More mags and more ammo before the fourth gun
But I would feel positively naked with only those three. Adequate, but it'd be like having only three sockets for your socket wrench. For the topic, I'll stick with those three, but now I'll stray from the guidelines.
Two is one and one is none. Fourth firearm would be another Glock 17. That would mean the dear wife and I would have the same pistol, and everything about them would be swappable.
Fifth would be another AR: neither of the two have to be totally mil-spec, but the idea would be to have all major components interchangeable, e.g., BCG, fire control group, charging handle, optic(s), certainly magazines, and so on. Then more extra mags and ammo.
Then I'd go for a beefier rifle. My preference there is .308 because, again, common NATO caliber, not because it's the best possible longer-range cartridge. I don't do precision shooting, so I'm happy with a reliable AR-10-type platform for that. Mine is more mechanically accurate than I can be, so it's perfectly serviceable out over 200 yards for game. And I like that the manual at arms and administrative handling is essentially the same as for an AR...plus nice 20-round PMAGs.
Number seven would be a smaller pistol. I'd keep it 9mm and choose a popular and more concealable platform like a G43 or a S&W Shield.
Number eight would be a Ruger 10/22, one of the "tactical" modular models that comes with an optic rail and likes those lovely little 25-round banana magazines.
Number nine would probably be a pistol caliber carbine in 9mm, maybe a takedown model, so that if bugging out is ever necessary we can cross-pollinate the same handgun ammo with something that offers a bit more accuracy and muzzle velocity.
That said, if the question were, "What firearms do you have that would never consider selling?" the answer changes a little. I've always been a 1911 fan and like the .45ACP. I have at least two 1911s that ain't goin' anywhere. Having long been a pump shotgun guy, I recently added a semi-auto...and wish I had sooner. A semi-auto 12-gauge would remain in the safe; I'm happy with the Mossy I bought, but I could sacrifice it if stepping up to a classy Benelli M4. I also like to have a really small, flat pocket pistol, one that can tuck away into business slacks when necessary; so I'd keep one of those. There are times when you just can't dress around the gun. And even though I'd keep the AR-10 platform I have, it's really setup more as a "tactical" rifle than a hunter. I'd stick with .308, but I'd keep a bolt-action rifle with good middle-range glass, like 3x9.
So, yeah. It's like potato chips. Just a few won't do. You're out of luck if you've got a decent selection of Phillips-head screwdrivers but what you need to work on are standard slotted screws.