We have four, one HD TiVo, two standard def TiVos and one HD receiver-but, every time I talk with D*, the rep tries to get me to reactivate my "office" receiver, which would be five-I think they'll give you as many as you want to pay for, and as many as your multiswitches or "array" of dishes, LOL, can support.
We watch our 50" Sony SXRD set in our family room adjacent to the kitchen more than any other TV in the house, so we put both the HD TiVo and the standard def TiVo (connected to a whole house modulator) on that TV. Because I wanted to also connect my TV's internal ATSC tuner, and the HD TiVo's antenna, that meant I had to have SEVEN RG6 coax lines to that location. (four TiVo tuners, two terrestrial tuners, and one back line for the modulator to connect to the video hub).
I had thought I was smart for prewiring four lines when we built the house only five years ago. Not.