Montreal stadium "McGill stadium" aka it's corporate name "Molson stadium" has a capacity of only slightly more than 21,000 and a bit more due to the completed renovations. So if they draw anything more than 21,000 you can consider that a capacity crowd.
The other stadium "the big-O" has a capacity of almost 70,000 but fans and players don't like travelling all the way to the east end of Montreal. The field is below ground level and it is always cold and damp place to play.
When the big-O was Montreal's regular stadium they had very low attendace, roughly 10,000 per game. Then for one game the big-O was not available and they used McGill stadium and to their surprise the drew 21,000+ fans. They wondered how these fans came out of the wood work. Maybe because the McGill stadium was closer to downtown, closer to the downtown action for the fans before or after the games.