Apple Remote Desktop Screen sharing to 20 computers get Black screen

We can use ARD to share the screen from the admin to 5-10 computers in a lab. When we try to share to the whole lab (20 units), we get black screens on the lab units.

Is this a limitation of our network or the software? A handful of any random iMacs work, the whole lab doesn't. I've reset all the connections with kickstart, removed and added all the iMacs in the admin console.