In this time of peak firearm and ammo sales I find 270 win and often 243win available in factory ammo.
Here's my minimum
AR 5.56 with an 22lr upper. This covers a lot of ground itself.
30-30 win. I like it and ammonis common
308 win or 30-06. Nothing needs said here but a person needs a popular 30cal rifle.
Optional would be an 44mag or 45-70 levergun if you were into a hobby of shooting and reloading and casting your own bullets. This is hobby status and not necessary.
If a person just didn't care for the 30cal rifles I would 100% have the 270win in that slot because it's availability in many rifles and ammo is very common.
Thats really it. 4 or 5 do a good job of taking care of everything. Wish I could whittle it down to that myself. Throw in a shotgun and an revolver and pistol and your truly outfitted.