Hi Henrik, there is not hard limit. Watch2Gether was initially designed for smaller groups but things have been optimized for larger groups as well. However, i would say that 400 is scratching at the upper border of whats possible. It could be that the browser becomes slow and unresponsive. More optimizations are on the way but probably not for the superbowl 
Mabye you can break it up in 3-4 rooms?