Webcams tend to use a lot of USB bandwidth and separating the cameras onto different USB host controllers (i.e. 3 in front, 3 in back or 2 in front, 4 in back etc) can potentially help reduce the bandwidth load on each controller allowing your computer to read more manycam at a time.
Please try distributing your cameras between different USB controllers by connecting them to the front and to the back of your system unit.