And as I mentioned, just my opinion of course. But I have had or used .308 and 30-30 hunting rifles, AR15 / M16's, shotguns and several calibers of handguns.
If I had one firearm in a civilized environment I would want a .40 or .45 auto with at least 2 x 10+ round capacity magazines, 4-5 inch barrel.
If only one in a apocalyptic situation I would want a SBR (Short Barrel Rifle) AR15 in .223 / 5.56.
Two weapons, I would want the same handgun and a 16.5"+ AR15.
Three I would add the shotgun.
Four I would add but have trouble deciding between a semiauto .22LR or a .308 or 30.06 hunting rifle.
Five would be the handgun, AR15, .22LR, shotgun and "hunting" rifle.
In those expanded scenarios the hunting rifle would really be purposed as a sniper rifle, as I would rather engage threats at a distance rather than < 100 yards. And those would be more likely to be one shot, one kill weapons. I would also assume that I would have at least one family member / friend manning the rifle, one on the AR15 and one on the shotgun, for the varying ranges and levels of defense.
If ammo is a concern, and I was trying to limit firearms based on that, I would stick with 9mm. Although I have no statistics to support this opinion, I'd be willing to wager there are more types and availability of 9mm chambered firearms out there than any other.