A Mediation Server is a necessary component for implementing Enterprise Voice and dial-in conferencing. The Mediation Server translates signalling and, in some configurations, media between the internal Lync Server infrastructure and a public switched telephone network (PSTN) gateway, IP-Private Branch Exchange (PBX), or a Session Initiation Protocol (SIP) trunk.

Mediation Server Scalability

A co-located Mediation server scales to a maximum of 226 concurrent calls. If the call volume exceeds this maximum number a dedicated Mediation server can be deployed. A dedicated server scales to, depending of the hardware and the ration remote vs. internal users, to a maximum of 1,200 concurrent calls per server.

For full details refer to the following Microsoft article: Capacity planning user model usage for Skype for Business Server.