This is a question I get asked a lot by customers (last week being the latest) and the answer really depends on which platform you are running on. There is a support policy in Books Online that states that we only support a maximum of 10 mirrored databases per instance on 32-bit systems, however in the 64-bit world we don’t have any such limitations. The limitation in 32-bit systems is due to the number of worker threads required for mirroring sessions and the fact that we are limited to a finite amount of memory for thread allocations.
However, these restrictions do not apply on 64-bit SQL Server systems and the SQLCAT team has just released a whitepaper showing that it’s possible to mirror in excess of 100 databases on a single SQL Server instance !! The article also has information to help you calculate the value of max_worker_threads and max server memory appropriately.
The article can be found here and is definitely worth a read if you plan to mirror multiple databases on a 64-bit instance of SQL Server: