This calculation for the number of RTL SDR receivers you need is only applicable if you want to use SDRTrunk as your preferred P25 decoding application. The approach this software takes is to decode the control channel and all of the voice frequencies simultaneously. So you need receivers capable of receiving all of the target site's frequencies at once. Because 1 dongle can only tune about 2MHz worth of spectrum (say, for example, 851-853MHz), you need multiple dongles to span the range of frequencies used by the site.
Other applications work differently. DSD+ Fastlane works more like a traditional scanner, in that it only tunes one channel at a time. This has advantages and disadvantages over other applications. There can be some advantage to using two separate dongles simultaneously, with DSD+ FL, particularly if you're monitoring a busy site and want talkgroup priorities to work especially well.
