Please take a look at our Amazon EC2: Quick Start Guide which shows the options you have available.
From here you can see the supported instances and pre-built AMIs which include the C3 and HS1.
More detail on the the instance types can be found on the Amazon EC2 Instances page which includes details on Cluster Networking.
C3, I2, CR1, G2, and HS1 instances support cluster networking. Instances launched into a common cluster placement group are placed into a logical cluster that provides high-bandwidth, low-latency networking between all instances in the cluster. Cluster networking is ideal for high performance analytics systems and many science and engineering applications, especially those using the MPI library standard for parallel programming.
You may also want to check out CloudFront. Please see our article below.
How to integrate your Wowza EC2 instance with CloudFront
I recommend asking the Amazon support team about the bandwidth you will get out of each instance as the number of connections is usually limited by bandwidth.
Using this number you can calculate the number of clients each server can potentially support based on the bitrate of the streams you’re looking to provide.