If you're recording on the web using a multi-channel audio interface or a multi-track mixer to control your sound input, connect your microphone/s to channels 1 and 2. Your computer browser can only detect audio only from these first two channels. All other channels will be undetected by your browser.
You can also change the interface's output to use mono or stereo sound.
Additionally, you can record on the Mac app with additional channels beyond 1 and 2 on your audio mixer.