How to record 4 instruments(2 vocal and 2 guitars) in Audacity 3.7.7 with Windows 11 and Focusrite 18i8 interface?
Not many people here have been successful. 
(I’ve never tried it.)
You may have better luck installing the Audacity version with ASIO. Your Focusrite has ASIO drivers available if they are not already installed.
It that doesn’t work you may need a DAW application. They are designed from the ground-up for multitrack recording & mixing. Your Focusrite may have come with light versions of Pro Tools and Ableton Live.