I think it’s just a number assigned to game accounts in the order that they were created.
With a number in the two thousands I’m assuming you played the beta or pre-registered for the game.
Mine is #4579 and I pre-registered.
I wondered this too, I think it’s just random. I have a lower number, 1765 than my bf, but signed up earlier.
I played the beta and am a higher number than you. It’s just random.
It must be random as I started in June and mine is 0398