Why are only 6 of my unattended computers showing up in my list for Assist?

Why are only 6 of my unattended computers showing up in my list for Assist?

I can only see 6 of my unattended computers in my list for Assist.  Everytime I add a new one, one of the others is removed.  Can someone explain this to me?  Am I doing something wrong?

Thanks,
Lezlie