The "Max SSC" is the number of concurrent users of Connect. If serialised, one user can connect at a time, but you cannot have more than one if your Max SSC entry is at 0.
Transfer Desk only requires SunSystems to be serialised, it is not tied up with an SSC serialisation. SunSystems needs to provide a tool to import /export data as part of the core product, but all the additional functionality provided by SSC or to run an import directly from SSC requires a serialisation for SSC; otherwise it won't work, and depending on the maximum number of users allowed, is the number of users that can run SSC at the same time.