What is the maximum number of on-camera speakers that can be supported?

Studio supports up to 8 concurrent speakers in a webcast at a time.  (Note:  There can be more sources, up to 12 using the Producer tool, but 8 max is recommended for best performance.)  

By default, an active speaker will be enabled, so the person speaking will appear in the largest view within the media player.

Once that threshold is reached, additional speakers will not be seen in the media player until they begin speaking and are switched into the active speaker position.

The above limitations apply to 540p and lower resolution webcasts. 

Please see the following Articles for information on restrictions for 720p and 1080p.

How many live sources can I use in a 720p Studio Webcast?

How Many Sources Can I Add to a Scene in a 1080p Webcast?