When I had the time to play regularly before, I had up to four balls.
Triton Heat 2 Pearl - Thick heads, clear backend
Chaos Black - Sanded. For those times when the mechanics are generous with the lane conditioning oil
Rage - Heads that have broken down with minimum backend
Lightning Storm - Deserts need only apply
Of course, all that was when I played three to five times a week. Now, should I get a chance to bowl and meet friends, I only bring two. And use one.
I've always been a "lazy" bowler who didn't like to switch balls. I prefer to adjust my release.