The standard sketch transmits 12 bytes of data, plus 9 bytes of overhead, at the rate of 49.23 kb/s, at intervals of 55 s approximately. Doing the maths, that means one transmitter occupies the radio channel for ~3.4 ms every 55 s, or 0.0062% of the time. 40 transmitters will together occupy the channel for ~0.25% of the time.
The standard sketch caters for an external temperature sensor plus a pulse count. Editing the sketch to remove those, assuming they are not wanted, will reduce the data to 6 bytes, it will transmit for 0.975 ms, or 0.0017% of the time. All 40 will occupy the channel for 0.07% of the time.
Provided that transmissions are evenly spaced, there is little chance of packet collisions. But as we know, there will be small speed differences and it is inevitable that collisions will occur.