I suspect you’re right – you’re hitting a fundamental USB 2.0 limitation. USB has 4 transfer types: control, bulk, interrupt, and isoch. http://www.jungo.com/st/support/docum…
Each had different characteristics and goals. Webcams and audio devices use Isoch, because it’s the one transfer type that allows reserving a specific percentage of USB 2.0’s 480 Mbps max throughput put host controller.
So with isoch devices, you can only connect a certain number, before these reservations will have taken up the schedule.
The Terminus Technology FE 2.1 hub controller in the USB2-HUB-AG7 is multi-TT, but that should only be relevant for routing USB 1.1 through the hub, and I’m assuming these are USB 2.0 webcams.
I don’t know what your goal is for total number of webcams, but I suspect that hardware upgrades would be the most reliable way to get there – specifically, each USB host controller has its own 480 Mbps bus, so if you can split the webcams between several host controllers, you can reliably avoid any issues with throughput or reservations.
But this is definitely an interesting scenario. Thanks for asking and hope that little bit of background helps!