Your Help seems to indicate you support an unlimited number of Custom License Fields. Is that true?
Yep.
Also are there performance considerations that we should be aware of if we decide to use so many Custom LIcense Fields?
Well, the data has to be downloaded (and cryptographically signed) as part of an activation. So, if I were you, I wouldn't put big blocks of data (more than 1KB per field) in the custom license fields. Or else activation will slow to a crawl.
But, if you're using custom license fields as designed (namely, to control flow in your app & plugins) then you shouldn't have any problems.