Many people will ask "how many sensors will I need?" The answer, as I've said before, is an unsatisfying it depends.
Indoor and outdoor applications differ, and even the type of terrain matters. Transmitter characteristcs matter, as well as antenna characteristics, and sensor placement. See thread on geolocation accuracy for a more detailed list of dependencies.
The Sensor Placement and Optimization Tool (SPOT) is a great tool for planning sensor networks. It will help you to determine how many sensors you will need, as well as where to place them, after some work in configuration of your scenario.
At a minimum, you wil need 3 sensors for TDOA geolocation, and 4 for Received Signal Strength geolocation.
How many more sensors you need on top of the minimum is a matter of planning.