It’s all most people need. Recently this question has come up again because of DISH’s new Hopper 3 which lets you run 7 clients at the same time. DIRECTV’s Genie doesn’t have that same ability, at least not yet. See, 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, DIRECTV-Ready TVs, other DIRECTV receivers or DVRs, RVU 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 16 DIRECTV things (like receivers, DVRs, RVU TVs, whatever) you’ll have problems.
So, here’s the bottom line: Regardless of what you “can” have, you probably only want to get 3 clients.