Hi,
Thank you for posting in Symantec community.
I would be glad to answer your question.
By default SEPM keeps 1 copy of the latest database backup.
To initiate on demand backup navigate through the following path:
Start > Programs > Symantec Endpoint Protection Manager > Database Back Up and Restore
Select the database backup option.
If there is a disk space issue then can refer the following article:
How to modify the Symantec Endpoint Protection Manager (SEPM) Database Backup and Restore Wizard backup location
http://www.symantec.com/docs/TECH103675