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.
It doesn't matter what jersey number Mbappe wears with his new club, what matters is his contribution to the team with beautiful goals scored from his feet and head. Real Madrid has honored Modric for one more season, even though this season he has contributed less to the team due to rarely getting playing time, his presence is really needed in the squad to be a mentor for young players. Number 10 will be Mbappe when Modric leaves, he will have to wait for that moment to come. Now he must focus on helping Real Madrid to defend the La Liga and UCL trophies. The opportunity is wide open to win his first UCL trophy because Real Madrid has always been the favorite team to win the competition.