Number of Site Collections in a Content Database:
There are no restrictions.

Note: A content database can scale well to 50,000 site collections. There’s no magic line at 50K, but you may start to notice some overall throughput degradation as you go beyond 50,000.

Size of a Content Database:

There are no size restrictions.

Note: A SharePoint content database can get substantially larger than 50GB. Microsoft IT provided guidance of ~ 50 GB for WSSv2 / SPS2003 content databases – this guidance came from the need for quick recovery of documents and site collections. Properly configured WSSv3 and MOSS 2007 installations should be able to handle TB sized content DBs.

via SharePoint Product Group

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