Sorry for the late reply. The maximum number of stacks is 25 at which point you get a Title "Chew Toy" which does nothing at the moment. However, this whole feature is getting removed in the next update and your Will Power Points will be refunded. I am currently working on a large Werewolf Village and the relationship with Werewolves is being reworked completely.
I should also note, that the attack of Werewolves increases based on the number of children they have as well, there is currently no cap on this damage (it will be capped later, mostly likely in the next release).