Hi,
Thank you for posting in Symantec community.
I would be glad to answer your question.
Embedded database supports 5000 clients per SEPM.
SQL database supports 80,000 clients per SEPM for SEP 12.1. In SEP 11.x version it's 50,000 clients per SEPM.
I do agree that it may happen database is able to support more than 5000 however it's not recommended by Symantec to use Embedded database to support 5000+. There must be some testing prior to come to the conclusion of 5000 clients per SEPM. Suddenly it may happen database is corrupted or policies are corrupted etc.
You should use SQL database as you are going to add few more clients in the network.