Well if you want to capture multiple videos from a Mac and potentially mix them here are some applications to try:

1. Capturesync. Best Software has this, in experimenting, it is nice because it puts all of the video into a single file. Since it is recasting files it is CPU intensive. Works fine with three uhd feeds and five hd feeds at 5 fps on a MacBook Pro (2014). 
2. ScopeBox. This monitors feeds and then turns it into video. This has a nicer interface, but doesn't scale beyond a few inputs and seems to hang.
3. VLC. Remove audio is one important thing and VLC does this pretty easily. 

Some standard dimensions:

1. Logitech C920. 1080p30
2. Logitech Brio. 4Kp30