For the most part, bandwidth is usually the limiting factor. As rule, consider this formula as a guide:
(stream bitrate * max concurrent clients) < server bandwidth
Other dependencies include how the workflow is configured. For example, if you’re using the Transcoder or nDVR AddOn, your publishing and packetization server load will increase.
While limited to RTMP, we do offer a load test tool that is very helpful in estimating load capacity:
How to get Flash RTMP Load Test Tool
Can you add more detail? Please describe your objectives and/or what you intend to provide and to what scale (how many viewers) do you anticipate?