It is normal that microphones are mono. For any stereo sound you need two distinct audio channels, so a left and a right microphone to pick up two slightly different versions of the audio.
(When having a stereo audio source i.e. an usb mixing console, you'd first need to have that stereo Windows. The popup at 4.png then will show the possibility to select 2 channels. Next in OBS at the Advanced Audio Properties at 2.png you'll uncheck Mono.)