I think I have 11 vassal duchies (5 in Ireland, 2 in Scotland, 3 in England, and 1 for the NE islands). Every time the Grand Duchies, especially the Scottish ones, rebel, I try to take a county to create a new duchy, figuring that a bunch of weak dukes will be less likely to challenge me than a couple strong ones. No more Grand Dukes holding three duchies, that is a painful civil war to fight.
This strategy has been full of fail, because the weak ones rebel anyway. I was revoking ducal titles and nominating one of their county heads to run the duchy, but I've done that so many times that there are no vassal counts in these duchies. Since the traitor will still be in charge of all 3 counties and I can't revoke each of those because you can only revoke a traitor's title once without your other vassals getting all frisky. Thus, revoking the title is meaningless. They just sit in jail until they die.