What happens to 5.8 GHz signals when too many transmitters are in a confined space?
Your system doesn't offer 40 non-overlapping channels.

The bandwidth of an analog video signal is around 10 MHz (I couldn't find the bandwidth for your type of transmitter, could be 8 MHz, could be 15 MHz), but as you can see ch 1 band FR1 and ch 8 FR2 have a 1 MHz spacing between carriers: the channels are significantly overlapping.
If two emitters use two overlapping channels, a receiver that receive them with the same power won't be able to demodulate anything correctly. If a receiver is much closer from an emitter than from the other one, the image quality could still be significantly degraded.

And even if the channels are not strictly overlapping (spacing between carriers > signal bandwidth):


a radio transmitter always send some energy above and bellow its set channel, more so if it's a cheap device (crappy radios) and/or a very compact device (less room for filters)
a radio receiver always received some energy from above and bellow its set channel (it's the receiver selectivity, rarely specified for consumer products), more so if it's cheap and/or compact


That's why you have to space the channels that are used simultaneously if you want to receive each of them with an acceptable quality.

Looking at your table, each "band" is in fact a "frequency plan", a way to arrange 8 channels in a non overlapping way (19 MHz minimum spacing) in a set frequency band of ~5.6 to ~5.9 GHz. I assume the different frequency plans were designed for interoperability (eg. maybe FR4 match the way a popular device manufacturer) and for legal compatibility (eg. FR3 has some channels around 5.9 GHz, and might be forbidden in some countries; FR1 doesn't).

Assuming you're the only one using videos transmitters in a set location (if not you'll have to coordinate with the other users), you should pick the same "band" setting for all your hardware, and select a different channel for each transmitter/receiver pair for a total of 8 usable channels, like it was suggested to you.

If 8 is definitely not enough, by carefully picking sufficiently spaced channels in several "bands" you might get a dozen of usable channels, but not 40.