He may renew his contract, currently expiring at the end of 2024, while Sainz will probably leave for AUDI.
The rumour is said to be like Leclerc has signed a deal of 2+3 years with Ferrari. There is more to that rumour also. Sainz is also reported to be after a similar deal with Ferrari.
I wouldn't be surprised with Leclerc's signing a contract extension truthfully. But I don't think Sainz will extend his contract that long. I also expect him to go to Audi when they enter the sport officially. Sainz's current contract expires at the end of 2024 while Audi are said to be entering Formula 1 by 2026. For only the time between Sainz would continue at Ferrari I think.