As many as the memory allows (and up to 1024, but was nowhere close that number), and I think that wasn't the problem here - but it seems that once the canvas has been drawn on for long enough, the new users just can't get the previous image. I'll have to refactor the initial loading code, at the moment it naively passes all the previous drawing operations to the client with no regards to the limits.

Sorry about this, hopefully I'll be able to fix these problems soon.