What would be the maximum number of concurrent Voice calls that can handle take from a single mediation server?
1) Standalone mediation server.
2) Collocated with FE server.
It depends on the number of servers configured in the pool
The Number of Video conferencing, voice calls that can be hosted on the number of servers depends on what other conferences like IM, desktop sharing is used in the organization as well.
Here is the calculation for video conferencing to be hosted on Front End Server.
This from the TechNet article – Scenario-Based Capacity Planning-
Can we use DNS Load Balancing for Mediation Server collocated with existing FE servers?
You must deploy DNS load balancing to support Mediation Server pools that have multiple Mediation Servers.
For details, see the Using DNS Load Balancing on Mediation Server Pools section of DNS Load Balancing in the Planning documentation.
Mediation servers should use only DNS load balancing according to Microsoft recommendation:
If you want to deploy multiple Mediation Servers in the pool in order to provide high availability, then select multiple computers pool option.
MVP – Exchange Server