Does anyone know of any max number of WP multisites or performance issues with having many?
Well, WordPress.com is a single installation of WordPress multisite (one extremely well optimized, cached, and load balanced installation, but still just one). I just checked, and they run over 19 million blogs there, with better performance than any site I've ever built.
Obviously, your server(s) and environments will need to grow with your site. I vaguely remember hearing somewhere that you will max out a single mysql database in the tens of thousands of sites; at that point (well, hopefully well before it) you'd have to move to multiple databases and use a plugin like HyperDB (which is based on the code used by WordPress.com).