Hi,
Thank you for posting in Symantec community.
I would be glad to answer your query.
Embedded database supports upto 5000 clients per SEPM.
SQL database supports upto 80,000 clients per SEPM in SEP 12.1. For SEP 11 it's 50,000 per SEPM.
For 500 clients I would recommend to go with Embedded database if not planning for failolver & loadbalancing. Only replication is possible with Embedded database.
With SQL database you can achieve failover & loadbalancing as well as replication.
Later on it's possible to convert Embedded database to SQL database as well.