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.