Yes, you heard that right!
The number of mountable databases for enterprise edition of Exchange has come down from 100 in 2010 to 50 in 2013.
As you can see, I have already mounted 50 databases on MBX1.
If I try to mount one more database, I get the error below, which is self explanatory.
This is all to do with the fact that a 2013 mailbox server handles a lot more load than 2010. On top of the extra load already in place, each mounted database takes a fair share of memory & CPU resources.
Hence, in order to have a good performance from a 2013 mailbox server, the Exchange Team has limited the number of mountable databases to 50. This may give Exchange architects & administrators some headache when they move to 2013, especially with very large Exchange 2010 deployments with multiple copies of a database.