Mbappe will wear number 9 before moving on to number 10, which is currently owned by Modric. Modrid got a one-year contract extension. I expect him to depart the club next season, and Mbappe can take number 10 from Modric. This is similar to when Ronaldo wore the number 9 first.
Modric is already in his ending days at the club that we all expected him to leave after the season but Madrid felt he is still worthy and gave him an additional year and who knows maybe his expertise might be needed. Wearing the no.9 shouldn’t be a problem for Mbappe especially if there is a player wearing the no.10 already and he is more experienced. I wouldn’t want him to wear No. 10 at the club i want him to wear no. 7 like his idol Ronaldo. When Ronaldo joined Madrid he also started wearing the No. 9, by the end of this season i expect him to be given No.7 instead of 10 because it looks more like a signature number to him.