Few people know that two transmitters when transmitting can produce a third frequency. This is known as IMD (Intermodulation Distortion)

Instead of discussing this, I am merely going to provide some links for further reading, then just do the hard work and give you the answers.

One REALLY good tool does all the work for you and that is the Channel Calculator by LT FPV here.

You will note that above calulator will only allow 5 pilots due to IMD, however some channels are less prone to IMD which is why Multi GP have gone with the channels for there 6 pilot configuration.

I want to also note, that if you have one quad close to receivers, and another far away, you will still have issues. Distance apart should be another factory to consider

AU LEGAL FREQUENCIES - See Chart here

6 Pilots - Not enough Frequencies
5 Pilots - Not Enough Frequencies
4 Pilots - 5740 F1, 5780 F3, 5820 F5, 5860 F7 OR 5725 A8, 5765 A6, 5825 A3, 5865
3 Pilots - 5740 F1, 5800 F4, 5860 F7
2 Pilots - 5740 F1, 5860 F7

US LEGAL Frequencies - More info here
(Using 5645 and 5945 in the USA could cost you a hefty fine... stay within 5650-5925 MHz

6 Pilots - 5685, 5740, 5780, 5820, 5860, 5905
5 Pilots - 5685, 5760, 5800, 5860, 5905
4 Pilots - 5685, 5760, 5860, 5905
3 Pilots - 5685, 5800, 5905
2 Pilots - 5685, 5905

ALL FREQUENCIES 

Please Note Legalities in your own country, check your country laws before flying in these frequencies

6 Pilots - 5645, 5685, 5760, 5805, 5905, 5945
5 Pilots - 5645, 5685, 5760, 5905, 5945
4 Pilots - 5645, 5740, 5820, 5860
3 Pilots - 5645, 5820, 5945
2 Pilots - 5645, 5945