This function will return the number of audio recording sources (like microphones, etc...) currently available to your game. So, if you have a return value of, for example, four, then you will have audio input on the sources 0, 1, 2 and 3, with one of these values being that which you use to start recording from using the function audio_start_recording(). This value can change at any time as people plug/unplug microphones or other input sources to the device. Note that you can use the function audio_get_recorder_info to get information on each device connected.
if audio_get_recorder_count() > 0
channel_index = audio_start_recording(0);
The above code checks the audio recorders available and if there are 1 or more, it starts recording from the first one indexed (source 0).