Hi,
Replication enables data to be duplicated between databases one separate sites so that both databases contain the same information. If one database fails, you can manage the entire site by using the information on the database from another site.
A partner is a management server on another site with a different database and management servers. A site may have as many partners as needed. Each partner, or remote site, connects to the main site or local site, which is the site that you are logged on to. All sites that are set up as partners are considered to be on the same site farm.
Embedded database supports upto 5000 clients per SEPM.
SQL database supports upto 80,000 clients per SEPM.
If you configured replication & if one of the SEPM goes down then other SEPM should take of SEP clients
High availibility depends upon network configuration, Server Hardware, OS and failover etc should be in place.
Replication and considerations:
https://www-secure.symantec.com/connect/articles/replication-and-considerations
Video:
https://www-secure.symantec.com/connect/videos/replication-concepts-and-configuration