For 5.8GHz FPV systems, there are 4 main bands divided into 8 channels each. That gives 32 channels free to use. So, in theory, 32 pilots should be able to fly FPV at the same time. Unfortunately, this is not true.

First of all, there are transmitters, and there are transmitters. Better, properly tuned, will broadcast in narrow frequency range. Cheaper, or just not properly tuned, will broadcast on much wider frequency range. The same goes for receivers. They can listen "only" to their frequency, but also to others. The result is, that a transmitter can pollute other channels. This is why, general rule is then 2 pilots should not fly on adjacent channels. If pilot A is using CH1, pilot B should use CH3 and so on.

Channel frequency for band (MHz)
Channel A (FR1) B (FR2) E (FR3) F (FR4)
1 5865 5733 5705 5740
2 5845 5752 5685 5760
3 5825 5771 5665 5780
4 5805 5790 5645 5800
5 5785 5809 5885 5820
6 5765 5828 5905 5840
7 5745 5847 5925 5860
8 5725 5866 5945 5880

Second of all, frequencies assigned to channels can overlap with channels in different bands. Just look at the chart below.

5.GHz FPV channel frequencies

Not only frequency separation between bands B and F is smaller than expected, there might me overlaps, channel 4 is crowded and band E jumps high and low. Nightmare that concludes: you should not fly CH4 on bands A and F. Also better not to use CH4 in band B. Just to be on safe side. On the other hand, band E (FR3) is not overlapping with other bands. Also band A is quite safe as long as you are not using CH4.