How many databases you can mirror on a single server relies on many factors such as workload, the number of CPUs, the capabilities of the server. You need to test it with a production workload to see how many databases are feasible with your environment.

You will not lose data during a network outage unless you lose the principal database. All of the transactions not yet mirrored will be preserved in the log file until mirroring reconnects and the transactions are transferred to the mirror.

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