Let's get the obvious out of the way first:
Players who are still learning and developing their game (timing,
balance, release, et cetera), stick to ONE ball. Don't waste your
money. Learn to have a reliable foundation before you start relying
on equipment. Scores shouldn't matter when you're training anyway.
Now, as for what you need for an arsenal. Every seasoned bowler
would have their mix, but here's mine:
One ball conditioned for each:
1- Extremely dry lanes (no heads, dry in-bewteen boards
10-30)
2- Broken down lanes (patchy heads, oil build-up on the outside
boards)
3- Fresh lanes / average conditions
4- Heavily-oiled lanes
That's what I used to carry when I played competitively. Although
in actual usage, it was mainly the balls for (1) and (3) that I
used the most. 
I hope this helps.