Macbook pro 2018 - microphone shown as 1 input in Audio MIDI setup
This seems normal.

Looking at the tech specs for both the 2016 MacBook Pro and the 2018 MacBook Pro, you will see under the "Audio" section, that they have 3 microphones and 2 microphones respectively.

So, it appears that the 2018 model has one less microphone channel and this is what you're seeing.  As for audio input, I would look at getting a quality USB audio DAC instead of using the built in microphones