I too was wondering what the level cap was. 50,000,000 xp seems like a heel of a lot. I wonder if these levels are cumulative with the next game in the series. That way it feels like your career means something and not having to restart every time a new Halo game releases. Maybe even tie some type of loyalty rewards to it to encourage longer careers in the Halo franchise.
Max SR, total XP needed, and it's rewards