the number of channels - this must be either 1 or 2, and the choice will depend on the results of canDoMono() and canDoStereo()
Question: Is there some other pre-built JUCE class that writes multi-channel audio data to a single file? Or must I create a custom binary file writer to hold such data?
The documentation you reading must have some error. The WavAudioFormat supports more channels. (But annoyingly, I think there is still some limit like maximum of 32 channels in the JUCE code.)