Although I have owned guns of various calibers, I am now down to only a few. Got rid of or don't shoot odd or calibers like 7mmMag (nice round but pricey to shoot) and 16 ga. cause I found them expensive or inconvenient to locate.
I am now down to shooting .22, 9mm, .45, .223, 12ga and soon .308 once again.
.22 because it is cheap and you can get lots of quality trigger time for little money.
9mm for much the same reason as .22, just not quite as cheap but centerfire and all that goes with it. I also usually carry 9mm so I need to be proficient with my 9's.
.45 because this is one of my favorite calibers and one I sometimes carry (and would probable rely on if the SHTF.)
.223 because, like modifiedbrowning, I enjoy shooting my AR180B. This caliber is one of the cheapest centerfire rifle caliber you can shoot in semiauto rifle (only 7.62x39 is cheaper.)
12ga. because my HD gun is a 12 ga with all the tacticool stuff and I need to be able to shoot it well enough to do what it is suposed to do. On a rare occasion I might shoot 20 ga. but that, as I said, is very rare.
.308 is a caliber I used to shoot and really liked but I cleared out my collection and don't have anything in 7.62 right now. I am currently looking for a deal on either a FN-FAL or a heavy barrel bolt action. When I get one I'll be shooting .308 again.
I have always thought that limiting my calibers to only a few was a good thing but I have found that all the ones I shoot, I don't want to give up.