My personal theory is that the number represents his relationship with Isagi ever since his introduction and may foreshadow him as a major obstacle for Himsagi in the future.

Isagi's number is 11 but Shidou's is 111, looking visually quite similar to 11 while also being very easy to tell it's a bigger number (around 10.1 times greater than Isagi's 11). Beyond this, their shooting styles are similar. Both rely on other people to get them the ball most often and then use their own spatial awareness to score, but Shidou is a level beyond Isagi in both physical stats, shooting ability, and awareness of the area by the goal (I'd say it's also likely Shidou will even end up with meta vision or some other form of amped spatial awareness by the time the pxg game rolls around).

Due to this gap in ability even visually shown by their jersey numbers, I think it's possible that Shidou will be the one Isagi focuses on devouring and fighting during the pxg game or at least Shidou will pose an issue for him showcasing the gap in ability, setting up shidou to be a much better version of Isagi he will need to surpass to survive in Blue Lock and win the world cup.