How many mirrrored databases can I have on a server?
10 mirrored databases per server is the recommendation for a 32 bit computer WITH 4 processors or less. 64 bit computers have twice as many threads which means, in theory, that you can have twice as many mirrored databases. Also more processors = more threads = more mirrored databases.
|Number of CPUs||Default # of Threads||Mirrored Databases||Default # of Threads||Mirrored Databases|
|<= 4 processors||256||10||512||20|
Others are correct to point out that this is a recommendation, not a hard limit. What your server can actually handle depends on the architecture as well as the amount of other traffic being handled by the server.
Please note that this is merely an interpolation of the "official" recommendation originally recommended for database mirroring limits in SQL Server 2005 Books Online. This is not a statement of how many databases you should expect to be able to mirror on any given server. The actual number of databases you can mirror should be much higher than this and relies on many different factors.
For a more complete understanding of how to calculate the number of databases you can mirror on a given server, I recommend reading and investigating the information documented by the SQL Cat team: Mirroring a Large Number of Databases in a Single SQL Server Instance. As always, be sure to test your planned set up with a realistic workload.