Max number of joints?

What is the maximum number of joints in a skeleton that unity can handle?

Theoretically, infinite. There is no hard maximum limit. And unfortunately I can't tell you where you'll start to run into performance issues, because it differs from platform to platform. My best advice would be to just do whatever you're thinking about doing, and if you run into problems, break up whatever it is you're jointing into parent/child relationships, as that may relieve some of the stress.

How to find out how many joints in an animation?