Limitations of SQL Server Express

You can create user instances and have each app talk to its very own SQL Express.

There is no limit on the number of databases.

There are a number of limitations, notably:

• Constrained to a single CPU (in 2012, this limitation has been changed to "The lesser of one socket or four cores", so multi-threading is possible)
• 1GB RAM (Same in 2008/2012)
• 4GB database size (raised to 10GB in SQL 2008 R2 and SQL 2012) per database

http://www.dotnetspider.com/tutorials/SqlServer-Tutorial-158.aspx http://www.microsoft.com/sqlserver/2008/en/us/editions.aspx

With regards to the number of databases, this MSDN article says there's no limit:

The 4 GB database size limit applies only to data files and not to log files. However, there are no limits to the number of databases that can be attached to the server.

However, as mentioned in the comments and above, the database size limit was raised to 10GB in 2008 R2 and 2012. Also, this 10GB limit only applies to relational data, and Filestream data does not count towards this limit (http://msdn.microsoft.com/en-us/library/bb895334.aspx).

Another limitation to consider is that SQL Server Express editions go into an idle mode after a period of disuse.

Understanding SQL Express behavior: Idle time resource usage, AUTO_CLOSE and User Instances:

When SQL Express is idle it aggressively trims back the working memory set by writing the cached data back to disk and releasing the memory.

But this is easily worked around: Is there a way to stop SQL Express 2008 from Idling?

If you switch from Web to Express you will no longer be able to use the SQL Server Agent service so you need to set up a different scheduler for maintenance and backups.