When you are about to run MAP (Microsoft Assessment and Planning Toolkit) assessment it comes by default with a SQL Server 2008 R2 Express database. This has a limit of a database of 10GB. This is in many cases not a problem but in larger environments it can be.
My experience tells me that a 10GB database is good for keeping data for about up to 10 000 machines. If you hit the limit the best option is to use a fully licensed SQL Server which has no limits on the database size.
The second option is to divide the scans to say for instance different OUs and switch between multiple databases. This does not give a combined report in any way so it is definitely not the best option but it will get you there.
The latest version of MAP 6.5 can be downloaded from Microsoft Download Center.