There are those of the opinion that you can get by with just one rifle, and there are those of the opinion that you need more than one rifle to fulfill different needs in your life. MadOgre recently expounded on the "scout rifle" as an example of a "one rifle solution" that is viable in our modern society.
Personally I believe that you can do it all with one, or even "none" as the situation may require. Bows, arrows, spears, swords, knives, snares, have all been used for offense, defense, and hunting purposes long before the invention of the firearm, and are still used to this day. The human brain and will is the most important survival tool.
That being said, I am a product of the modern world, and I believe that the optimal solution for me is multiple firearms.
22 rifle and pistol
Big Game rifle, 45 ACP/9mm pistol
12 gauge Shotgun.
To explain my reasoning, I have a family, and while I won't ever carry more than two firearms at a time, that doesn't mean my family won't also have need of a firearm. The 22s are useful for taking small game, and teaching new shooters how to shoot. The 45 or 9mm is for protection, and the big rifle and 12 gauge are for hunting and protection.
That is the bare minimum that I would feel comfortable and still calling myself "prepared" by any stretch of the imagination. If it were just me, pistol, rifle, shotgun, would be enough to feel fully prepared, and if I needed to be very mobile, just the rifle.