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APPLIES TO: 
2016 2019 Subscription Edition
Exchange Server is available in two server editions:
Enterprise Edition: Can scale up to 100 mounted databases per server.
Standard Edition: Limited to five mounted databases per server.
A mounted database is a database that's in use (an active mailbox database that's mounted for use by clients or a passive mailbox database that's mounted in recovery for log replication and replay). While you can create more databases than the described limits, you can only mount the maximum number of databases that are allowed by the edition of Exchange. Note that the recovery database doesn't count towards these limits.
The server editions are defined by a product key. When you enter a valid product key, the supported edition for the server is established. For more information, see Enter your Exchange Server product key.
Notes:
You can use a valid product key to move from the Trial Edition (evaluation version) of Exchange to either Standard Edition or Enterprise Edition. No loss of functionality occurs after the Trial Edition expires, so you can maintain lab, demo, training, and other non-production environments beyond 180 days without having to reinstall the Trial Edition of Exchange or entering a product key.
You can use a valid product key to move from Standard Edition to Enterprise Edition.
You can't use a valid product key to downgrade from Enterprise Edition to Standard Edition or revert to the Trial Edition. You can only do these types of downgrades by uninstalling Exchange, reinstalling Exchange, and entering the correct product key.
Exchange Server versions
For a list of Exchange Server versions and how to download and upgrade to the latest version of Exchange, see the following topics:
Exchange Server license types
For more information about Exchange license types, see Exchange Licensing FAQs.