In contrast to what, apparently, many people believe, communication between satellites and GPS devices is purely one-way. Satellites send out a timing signal and GPS devices pick up this signal. Combining the timing signal from multiple satellites with knowledge of where the satellites are located in space allows the GPS device to compute its location.
The GPS device doesn't need to transmit anything and is completely passive. And the satellites don't receive anything from the GPS devices. Consequently, there is no limit to how many GPS devices can be used in the same area.