It depends on codecs and used hardware.
We do not have information regarding Amazone instances performance.
You need to install the software and make several tests to predict CPU utilization for 1, 10, 20 calls etc.
Amazon EC is a virtual environment.
We do not know how virtual characteristics of an instance are mapped to hardware CPU cores.
As we know, m1.large instance is equivalent of old Core 2 Duo CPU.
Perhaps it may handle about 50-100 video sessions without transcoding.
