I think we can all agree that one of the guns you NEED to have is a 12ga. shotgun - pump or semi-auto. I personally have a Remington 11-87 (semi-auto). If you like the pump guns, look at Remington 870. While there are many other brands out there, Remington is a great platform because they are so popular that everyone makes aftermarket accessories for them.
As for hunting rifles, I would agree that everyone should have a 22lr. Personally I prefer a semi-auto for quick follow-ups on scurrying varmints.
Now lets move to the 'big' guns. There are three in my personal battery:
My 280 - Updated!
Since you want a rifle(s) that are easy to get ammo for, I would recommend a rifle like this in 30-06.
My 260 by MCR
Here again, 260rem ammo might be difficult to get. So if you had a rifle like this in 243win or 308win it would be easy to get ammo for it. Matter of fact, this rifle started life as a 308 My 308 1k Rifle - The Evolution Continues!
My 300RUM by MCR
Finally, everyone should have a big cannon for shooting at BIG critters and waaaaaaaaaay over there. While 300rum is a beast, you might consider a 300winmag as 300winmag ammo is quite abundant.
This makes my personal answer to your initial question: SIX
In the alternative, you could cover this in FOUR: 12ga shotgun, 22lr semi-auto, carry rifle 30-06, prone gun 300winmag.