The number of video streams that a single Box can handle depends on the bitrate, resolution, and frame rate of the video streams connected to it and the number of CPU cores and RAM of the Box.
See the article How do I tune my Camio Box VM to match the number of connected video streams?
The max also depends on the activity observed in the scene itself. The maximum number of video streams increases quickly (non-linearly) as the bitrate and frame rate decrease, and each connected stream can have different video encoding settings.
See "How do I know that a Box can handle more video streams"