Canvas size seems too small. What is it 400x 400 pixels? I didnt actually count accurately, but that's like 150 32x32 size canvases. Hardly seems like enough. If its even as much as 500x500 I don't imagine its even possible for more than 250 accounts to contribute assuming a generous contribution of just 32x32 pixels per account...

Edit: just realized there's pixel coordinates which confirms a 500x500 sized canvas. Canvas@toast has over 700 users.