I have seen from 80 - 500 quoted, so take your pick. The pairing is almost infinite and the band allows for thousands of channels, so it's just the intermodulation, receiver rejection and guard bands that you would need to factor in to get a correct figure.
Not sure it's ever been tested in anger, but even the BMFA nationals with upwards of 200 competitors seem quite relaxed about 2.4GHz ISM TX use both on the flightline and in the testing area.
The main issue is more from powerful Txs on other channels, swamping the receiver and reducing selectivity to the point that the receiver will accept a control signal on virtually any frequency. This has been shown to lead to loss of control. but affects all low power radio systems, not just radio control.