SQL Database Mirroring Number Restriction
Per the official Microsoft documentation, there isn't a technical limitation on the number of databases you can mirror. The more you have, the more you will have to manage, obviously.

As noted in that link, mirroring is deprecated and will be removed from SQL Server at some point in the future. Microsoft themselves recommend against implementing a new solution using mirroring as a result. You should consider alternatives such as log shipping, which may give you the protection you need (depending upon your SLAs, RTO, RPO, etc.).