The number of cameras/mixers you can have is generally limited to the available resources on your computer. Currently, the number of File Descriptors available in the OS limits cameras to about 50. If you have many \textit{mixer viewers} you will probably want to use proxy mode whenever possible. Also, in the \texttt{Settings $\rightarrow$ Playback A} tab \textit{Video Out} section, uncheck \textit{play every frame} and choosing a Video Driver of \textit{X11} with \textit{use direct X11 render if possible} checked, will provide better performance.
The number of cameras/mixers you can have is generally limited to the available resources on your computer. Currently, the number of File Descriptors available in the OS limits cameras to about 50. If you have many \textit{mixer viewers} you will probably want to use proxy mode whenever possible. Also, in the \texttt{Settings $\rightarrow$ Playback A} tab \textit{Video Out} section, uncheck \textit{play every frame} and choosing a Video Driver of \textit{X11} with \textit{use direct X11 render if possible} checked, will provide better performance.