5 balls total. 1 urethane, 1 plastic, and 3 whatever the hell I feel like taking.

I take two 3-ball rollers, so that gives me five "regular" balls plus plastic. I know that it's total over-kill, but I figure why own the balls if you're never even going to consider bringing/using them.

I'll be bringing 3 to each match and practice now.
For matches on home turf and practices I'll bring: Scandal, Grudge Hybrid and the spare ball.
For matches at the alley where the lanes dry up fast: Swerve, Freakshow and spare ball.

1 Quick Response Ball
1 Slower Response Ball
1 Urethane

House, 2, longer hook phase ball and urethane. Sport, I bring three, surface/shine/urethane. It's infuriating to see guys who have bowled a house league for a decade show up with 6 balls and in most cases said balls do the same thing. Furthermore, these guys have all these balls, but won't move their feet or move left or right of the track. Nope, let's jam 5 balls up the track and hope one works.

Or they bring all 6, throw whichever one is the newest, and shoot 490 just so they have a story to tell about how bad the newest ball from ___ is.

5 one is urethane that is my spare ball. One that hooks alot three that hook less

I still take 6 no matter where I go. It's more of a habit than anything else, since invariably I will only use two a night. The one I've chosen and my spare ball.

I don't bowl in the same house all the time so it's good to have variety. The lane man is never always the same either in my regular house, so I always have a back-up plan just in case this is the "rookie's" first night treating the lanes!

One of the balls that never leaves my bag is my slow-response Saturn that DP3 sold me many years ago. Love this ball because of it's versatility, especially on light oil and dryish lanes. Oh yeah, and I never had it re-drilled. Started using it the day he sold it to me!

So its usually one hi-performance; two mediums; one light oil and a spare. And that one ball that I constantly change out of my bag, as an experiment.

Another thing to consider for this is how much you're throwing down on brackets, jackpots, eliminators, etc. each league night too. In some houses I bowl at those are extremely popular and competitive. If you want to get in them you better be bringing the best/most equipment you can. Don't want to lose $100+ instead of making a profit because you left a ball home that would be a better fit that particular night.

This used to be my reason for bringing a large arsenal every time I bowled.  Trunk of my car would have 4 double ball totes just in case.  I remember my father in the 70s bringing many balls for the same reason.  He would be bowling and call me over to get a ball out of the car.  Just never know, especially in the 70s where the conditions in the area were no where near as consistent as they are now.