hmmm, when I started finger-tip bowling, I could get about 10-12 revs. I wasn't too sure on how to get more, so for that time I just bowled like that
As I progressed and improved with fingertip, I got a few more revs and by the time I got to shield roll-offs in 2002 i had about 12-15 revs (it doesnt seem like much of an increase, but I can assure you it is).
The best way to get around the ball and increase your rev rate is to let your fingers and your wrist do the work. The best way to do this is when you get to your slide or step, as your hand is about to go past your knee, your thumb comes out first and try to tuck it in your hand. You should be holding onto the ball with just your finger for about .75 seconds (you should also still be behind under the ball, if not, its alot more difficult and you'll top the ball). As your doing this, its just a quick 45-90 degree turn of the wrist and you'll get the ball to rev.
Thats how I was taught how to rev the ball, and my rev count is pretty high now. I hope this helps in some sort of way, but if not, its best to ask a coach for help, as Crawf said.