Hi,
You can find the answer to this in the
FAQ.
The limit is up to ~300 Hz of total transmissions in the same wireless range on the same RF channel before collisions may begin occurring permanently. So say 75 4Hz ANT+ sensors, 600 0.5Hz Geocache beacons, etc, you can have an unlimited number of listening devices (unless they also transmit, in which case they will add to the over the air traffic).
Cheers