I'm playing a sacrifice build Witch Doctor, and recently added Mass Confusion with Devolution rune. However, I have yet to get any additional zombie dogs from killing a confused opponent.
Each time, I've opened with Soul Harvest, then hit Mass Confusion, then Acid Cloud. That usually kills most of them before my zombie dogs are in melee for more than a second or two.
Yet I always wind up with 4 zombie dogs. Is that the hard limit to the number available?