What to consider:

- Number of simultaneous users your application must support at peak load
- Data transmission speed at which you expect your users to connect (what percent of users will be on a 56 Kbps dial-up connection, cable/DSL, or a LAN?)
- Number of audio/video streams your application will support per user
- Limitations of the server hardware and Internet connection
- Target encoding rate of the audio and video streams in your application

Flash Media Server Developer Center