The numbers represent the sequence of the grit level of abrasives used to finish the ball. There are as many as was used to prepare the surface.

If you sand a ball 500 grit and then go to a 4000 grit pad, the surface will be more aggressive than if you used the full sequence, step by step, as in
500 grit, 1000 grit, 2000 grit, 4000 grit, as an example.

Also there are different compounds and polishes used by different companies.
All compounds or polishes are applied over a base grit level or sequence of grits.
Examples:
1. 500 grit, 1000 grit, 2000 grit, then polish.
2. 500 grit,  then polish.
3. 500 grit, 1000 grit, 2000 grit, 4000 grit, polish.

One can use virtually any "logical" sequence of abrasives and/or compounds.

Some balls are even polished, then are sanded lightly or firmly with another abrasive.

NOTE: Manufacturer's surfaces are, more often than not, very hard for the local enthusiast to replicate at home, even with a spinner. Manufacturers often use devices that we, at home, have no access to. So many people will change the manufacturer's surface to one they expect to work well for them AND which they know they can replicate.

A good understanding of surface adjustment is essential for the best performance on modern day bowling balls.  Dont be afraid to "tweak" the factory polish off of a ball.  Too many times I hear that " this ball is too over-under, I hate it"  Most of the factory polished balls, for a player with above average speed will be "over-under".  Just hitting the ball with a 4000 abralon pad will do wonders in smoothing out a reaction.  Experiment and see what works best for your game.