A. In the standard edition of Exchange 2007, you can have up to five databases spread over five storage groups. With enterprise edition, you can have 50 databases spread over 50 storage groups. This works out to the ability to have one storage group for each database, which is actually the best practice, allowing you to remove shared transaction logs between databases and eases recoverability. There's no reason to put more than one database in a storage group, and you should add new storage groups as you add new databases.
There is a maximum of five databases per storage group, the same as in Exchange 2003, but as I mentioned above, it's strongly discouraged.
There are circumstances where you must have only one database per storage group. If you plan on using Cluster Continuous Replication (CCR), Standby Continuous Replication (SCR) or Local Continuous Replication (LCR), the only supported storage groups are those with one database in them. If you want to use CCR, SCR, or LCR and have multiple databases in each storage group, you'll need to redesign your databases so you only have one database in each storage group. This isn't a requirement for Single Copy Clusters (SCC), which don't actually perform any log shipping replication because there's only one copy of the database.
See this page for Microsoft's official database recommendations.
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