I've had a few hundred in an empty scene before. Any simulation beyond 300 frames and the meshes started to interfere and become locked onto each other.
Basically, they'd "avoid" each other up to a point, but once past 300 or so frames, one mesh would freeze, and then any others that walked into its path would freeze onto the original frozen mesh. At the end of 1800 frames, I ended up with a huge mass of people meshes all bound up in the middle of an intersection, but the people on the outer edges of the paths were unaffected, and continued to walk like normal.