UPDATED FOR 2017
Here’s the simplest answer I can give you. If your Genie DVR is wide and flat, it only lets you stream to 3 HD boxes at a time. If it’s tall and triangular, it lets you stream 5 HD boxes and 2 4K boxes at a time.
It doesn’t matter if they are C61s, C51s, C41s, C31s, wireless Genie Mini Clients, other DIRECTV receivers or DVRs, DIRECTV-ready TVs, whatever. Three is the magic number for the older Genies. For the Genie 2, it’s 5+2.
What happens if you have more than that? They just don’t work, not at all. You’ll pay for them, and DIRECTV will let you do that, and you won’t be able to use them while other clients are in use.
So, here’s the bottom line: Regardless of what you “can” have, you probably only want to get 3 clients for the older Genie and 7 clients for the Genie 2.