Simple question, right?
Here's the simplest answer I can give you. The Genie DVR only lets you stream 3 things.
That means only 3 clients will work at a time.
It doesn't matter if they are C51s, C41s, C31s, wireless Genie Mini Clients, other DIRECTV receivers or DVRs, DIRECTV-ready TVs, whatever. Three is the magic number.
What happens if you have more than 3? DIRECTV will let you add up to 7 clients. You'll pay for them full-time but you can't use more than 3 at a time. Also if you have more than 15 DIRECTV things (like receivers, DVRs, RVU TVs, whatever) you'll have problems. This is due to the underlying technology that DIRECTV uses to communicate between receivers.
So, here's the bottom line: Regardless of what you "can" have, you probably only want to get 3 clients.
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