Your obvious solution here is to use a crystal per microcontroller! At least that way you're not trying to do something that may cause unreliability!

I do question why you want a number of microcontrollers for counting. How fast are you counting? How do they communicate with each other? Do they need to communicate with each other? I'm thinking it might be easier to use a micro more suited to your problem rather than use a number of them. If you can tell us more about what you're trying to achieve, then maybe we can offer a solution that gets away from trying to drive a number of micros off one crystal.

Too often we get caught in a 'mindset' of a particular solution. With some lateral thinking, a better solution can usually be found.