ANT works on a 2.4GHz, 0dB radio. The modules are designed to provide 30m line of sight operation. Actual distance will greatly depend on environment - walls, floors, water, bodies, etc ... all attenuate the signal and will reduce distance. If you design your own modules you may get better or worse distance performance.
One way to increase the effective distance is to using "hopping nodes", to send data from node to node until it reaches its destination. ANT has the capabilities to do this - however, to make this truly scalable will require a lot of work at the application level - ANT does not implement full mesh capabilities, but it does readily support fixed topology tree networks. The are 2^31 addressable nodes in an ANT network. How many you actually have will depend on what type of network you are trying to design. A network with 100 nodes should not be a problem.
ANT is very low power, you can even design sensors that run on coin cell batteries for years without needing to be recharged.
Do you have access to the ANT Dev Zone? There is a university program that will grant you access. There is a lot of information in the dev zone that will answer a lot of your questions in more detail. Also check out the Downloads area - specifically the document ANT Message Protocol and Usage.
Sebastian
