Suggested

driver race number

Depends if you're using 3D or 2D and blobs or cars with the latter. Also I'd plan such a feature from the start with including offering a few number font styles and 2 or 3 colours depending if background is transparent only or can be done custom. Those custom numbers can appear in the drivers pictures, league driver leaderboard, for 2D blobs on them and the chosen style maybe if position is selected as well as it offers a much better visual identifier, as an option for 2D cars view and 3D as floater on all, the cars ahead and behind the own, only own, or off and would be the first 'decal' texture I'd try to get on 3D cars. For ingame leaderboard it depends as I'm not sure right now if there's room for those numbers in their own column or go for a solution like a toggle to switch position and car number in that column or a third option for the driver name/team column showing driver number and driver short handle/letter code and maybe flag mimicking real racing leader boards.