You are ordering EventMaster (which includes MessageMaster), or just MessageMaster, and you are wondering how many server licenses you will need for your environment.
Typically you need only one server. And this means one SQL Database will receive data from all clients. Even if you have multiple sites, it is typically enough to have one server in a central site.
However, if one of your other sites is very large (300+ clients), it becomes more efficient to install a second server at that site. That way, all of the clients in the large site will report directly to the server in their local site. The SQL Server database in the large site will then need to be configured to replicate with the SQL Server database in the primary site. This replication traffic is more efficient than individual clients sending information individually, and can also be configured to only replicate at certain times to avoid peak WAN usage.
You need to remember to have your clients point to the correct server (this is simply a registry entry). The multi-server setup treats the two servers as equal peers, since replication is two-way. The console can be run at either server or both, and searches against messages at either server will return the same results once replication has taken place.