How to take backup of repositories in Plastic SCM ? is taking a manual backup of database is enough ? will I be able to restore and start using the repositories in future if system server crashes ? Or is there any other method or interface to do this job ?
That would be covered by the chapter "Backup and restore" of the Plastic SCM (4.xw) Administration guide:
The backup and restore procedures are closely related to the database backend used in Plastic SCM
so yes, backing up the database seems enough.