There is no max. However there used to be and may still be a limit in the admin control panel to how many are displayed. There is a performance issue with 10,000 forums/subforums but thats any software. Any software can work around it a bit so yes there is a performance difference between software suites.
There's no hard limit, but I doubt with 10000 you'd get ideal performance or UI without making specific customizations. Consider the forum list which pulls data from all forums (and can actually display all of them) or interfaces where you need to pick a particular forum out of all of them (moving a thread).