If the system is a one-way radio channel using polling equipment, not more that 500 boxes are allowed on a single frequency.  

If the system is arranged as a two-way system that uses interrogation-response equipment, not more than 250 boxes are allowed on a single frequency. If the interrogation-response system uses a different frequencyNFPA 72 requires a specifically assigned frequency for fire alarm boxes. This allows the transmission from the box system to avoid interference from such things as radios used by fire departments, police, or other entities. than the alarm box transmission frequency, the number can be increased to 500 boxes.

All coded radio boxes require the frequency used to be monitoredIf the frequency is lost, a visible and audible signal is transmitted and received at each affected receiving system in the public fire alarm reporting station. for integrity. The frequency has to be used every 24 hours to test the system.

NOTE: This is an unformatted excerpt from our online fire protection training library.