As far as I know there’s no limit for MKV. DVD and Blu-ray are each limited to 32 subtitle tracks, so for those formats, you’ll need to prioritize which ones you really want. I’ve given some suggestions for how to do this in the README file, but really it’s subjective based on what your target is (i.e. I prioritize based on number of speakers, but if you’re targeting Europe, you may want to skip Korean and include Finnish). Some tools might complain when you go over 32 due to this limit, but I haven’t seen anything indicating it’s a problem inherent in MKV or even M2TS.