There's always a lot of responsibilities attached to players wearing a particular jersey number for a team. Real Madrid has its own iconic numbers which can't be worn by just any player because of how excellently players back that wore it performed. Bellingham chose the number 5 jersey because that was the jersey the Legendary Zidane wore in Real Madrid. Last I remember was Ozil putting on that number and when he left, Modric assumed mantle ship of which he played his heart out with it excellently just like Ozil did.
Remember when Ronaldo left in 2018 and because Perez and Ronaldo fell apart, he hurriedly gave the number 7 to Mariano Diaz, lol, who made a mockery of that jersey number, he was forced to leave the club afterwards and the jersey was kept. Vini back then was putting on the number 20, it was later when he was ready that Real Madrid handed it over to him and he has proved himself on it. Mbappe just got himself some higher level of responsibility now. Yamal took the number of Barcelona, he gat to brace up now because that's a lot of responsibility.