SMSQL needs to be installed on every server holding an SQL instance. SMSQL can be managed on each host individualy or you can install SMSQL on any Server of your choice and add all SQL servers into the GUI.
Besides SMSQL you will of course need SnapDrive on every SQL server. SnapDrive must be licensed as well.
Re: SnapManger for SQL needs to install on all servers that have SQL db's or just one one centralized server?