Multiple Databases and the SQL Server

When using storage arrays with SQL Server, it is recommended to use the VSS Requestor/Provider to take full advantage of highly efficient snapshots and replication. The VSS Requestor/Provider works with Microsoft Volume Shadow Copy Service (VSS) to coordinate snapshots with applications writing to the volumes. This allows the VSS Services to take consistent snapshots of any VSS-aware application, such as SQL Server.
NOTE: Microsoft recommends less than 35 databases involved in a VSS snapshot on x64 systems.

Also see Microsoft Knowledge Base article 943471.