If you want to estimate the number of basestations then assume a range of 20 meters.
The building were they are used has to be taken in mind, in a wooden construction the range is a lot better as in a concrete construction and in a building where a lot of metal fences and coated glass are used the range can drop down to 10 meters.
I areas to expect heavy call loads ( a lot of dect users together ) you can deploy upto three basestations but they need to be placed as close to each other as possible but do NOT put one on top of another, consider this as one basestation in your measurement.
A simple mind delivers great solutions