The 20P will support a single RF reciever, no more, since the system works as a fixed addressable type.
The best location for a RF reciever is dependent on construction and range. I have yet to really see many issues with a 'standard' install by mounting the RF reciever near the control. The best location would be highest as possible, and in conditioned space (2nd floor closet) and nearest the center of the house. Barring that, a first floor closet will work as well. I haven't had any issues with a standard house before, only when in larger installs, or trying to push the distance between a house and outbuilding.