As of April 2021 the max level is 409Real simple: After maxing your level (depending on how many new skills they add over time) right now it is 409, and after leveling up you can insert points into one of the three masteries as seen in the image below.
Edit: You also have to obtain all of the new skills they have added so far (9 of them)
https://imgur.com/RTe10qk