I can't speak for Windows instances, but I will presume that their base characteristics are fairly similar to Linux instances.
Your estimate for bandwidth usage is 100 simultaneous video downloads (I am not sure if you mean downloading the file or streaming the video - I will assume the latter). If we take a stream rate of 512kbps, you need about 51Mbit/s or 6.5MB/s.
EC2 instances differ in their I/O performance (which includes bandwidth). There are 3 levels of I/O performance: low, moderate, and high. Keep in mind, though, that disk I/O (i.e. from EBS volumes) also is bandwidth dependent. You can only really consider bandwidth within the EC2 network (as it will be completely variable over the Internet).
Some typical numbers to quantify 'low', 'medium', and 'high' (different sources quote different numbers for theoretical values, so they might not be completely accurate).
High:
Theoretical: 1Gbps = 125MB/s;
Realistic (source): 750Mbps = 95MB/s
Moderate:
Theoretical: 250Mbps;
Realistic (source, p57): 80Mbps = 10MB/s
Low:
Theoretical: 100Mbps;
Realistic (from my own tests): 10-15Mbps = 1-2MB/s
(There is actually a 'very high' level as well (10Gbps theoretical) but that applies only to cluster compute instances only).
A further point of mention is the degree of variation. On smaller instances, there is more variability in performance as the physical components are shared between more virtual machines. Regardless, you can expect around +/-20% variation in your performance (sources: 1, 2, 3). In your case (as per the assumptions/calculations at the top), you may need peak bandwidth of 13MB/s (double 6.5MBps, since disk I/O is also network limited). If you are transferring lower bandwidth content, you should be able to use an instance with 'moderate' I/O performance (see the instance types page), if your calculations result in a higher bandwidth requirement, you will need an instance with 'high' I/O performance. Simply streaming the data should not be CPU or memory bound, but sustaining 100 simultaneous connections will probably require at least a medium sized instance - and if bandwidth is a concern, based on the above, a large instance would be a safer bet).
I would recommend benchmarking the servers you launch to see if they meet your (calculated) needs. Launch two instances (of the same type) and run iperf
on each using the instances' private IP addresses - you will need to open port 5001 in your security group if you run it with the default settings). Additionally, most tests outside of the EC2 network show results of between 80-130Mbps (large instances) - although such numbers are not necessarily meaningful.
A CDN would be better suited to your needs, if your setup permits it. S3 appear to have a limit around 50MB/s for bandwidth (at least from a single instance) as per this article, but that is higher than what you should require (S3 does not support streaming). Cloudfront would be better suited to your task (as it is designed as a CDN) and supports 1000Mbps=125MB/s by default (source) with higher bandwidth available on request and can stream content as well)
Best Answer
Technically, given the units in your question your math is wrong. However, it is more semantically wrong as it could be construed as right depending on how one defines the units.
The big confusion comes down to the unit being used. A kilobit versus a kibibit. A megabit versus a mebibit.
See the Wikipedia page on Data rate units for more detail.
Additionally, I am going to assume on the wire data rates for the stream. If your media stream -- what the encoder is outputting, is 1,024 kilobits per second the on the wire data rate (RTP, UDP, IP, Ethernet overhead) will be larger than 1,024 kilobits per second. You must take into account overhead if you want to be exact in the real world. However, the example below is to illustrate the math regardless of encoder output rate.
For our example, I assume the on the wire data rate is 1,024 kilobits per second, meaning encoder output is less than 1,024 kilobits per second.
A 1,024 kilobit per second on the wire data rate is 1,024,000 bits per second. A 100 megabit per second link is 100,000,000 bits per second.
100,000,000 bps / 1,024,000 bps yields 97 possible "streams/data rates" of 1,024 kilobits per second on a 100 megabit link.
Like my physics teacher always stammered: Units! Units! Units!