The number of dongles is only limited by capture settings on each dongle and available bandwidth on your computer’s USB root hub.

Every computer has at least one USB root hub. USB 3.0 has Gbit/sec of max throughput available for the sum of all data consumed by all connected devices connected to the root hub. However, testing shows the actual throughput is about 2.8 - 3.0 Gbit/sec or 360 - 380 Mega Bytes per second (MB/sec.) The frame size, frames per second and color space for the captured video all effect the transfer data rate from each USB dongle to your computer. For example: using YUY2 color space and 30 fps you can fit two or three 1080p dongles on the same bus 125MB x 3 = 375MB. This assumes optimal USB throughput - after that you may start dropping frames on the incoming video.

You also need to consider other USB connected devices such as displays or network interfaces that will also compete for bandwidth as well as built in devices such as touchpads and webcams that are internally connected to the USB bus.

Other considerations:

• Some high-end systems have two high performance USB root hubs - many do not. Check your system specs.
• If your system has a USB 3.1 port then you will have 10 Gbit/s available bandwidth on the USB root hub. Thunderbolt 3 will have 40 Gbit/s (note: the full 40 is not delegated to USB devices.)
• USB 3.2 has 20 Gbits/sec (3.2 devices will eventually become available on the market))
• USB 4.0 specs are now being finalized which is rumored to have 40 GBits throughput.

The chart below shows how much data each capture setting will consume on the USB root hub.   

[1] Uncompressed lossless video stream