I use EXWM and three monitors, so knowing my window numbers is crucial to my navigation. I used to use Ace Window to present a red digit at the corner of emacs screens with which number to press to get there. The two problems with this are that you can’t know your numbers until you invoke Ace Windows, and more damningly, you can’t get these numbers on exwm (non-emacs) windows, such as my ever-present browsers.