Maybe this if you want to rack them 
http://usb.brando.com/industrial-usb-16-port-hub_p01936c039d015.html
http://usb.brando.com/industrial-usb-16-port-hub_p01936c039d015.html
16-Port Hub: Up to 16 hubs are provided for USB connection like printer, digital camera, card reader, MP3 player etc
Fully compliant with USB 2.0 specification
Backward compatible with USB 1.1
Input: 100-240V AC, 50/60Hz, 0.5A
Output: 5V DC, 3A, 15W
Dimension: 169 x 118 x 38mm (approx.)
Weight: 310g
Package Contents:
Industrial USB 16-Port Hub
USB Cable
Universal AC adapter
again 5v...... cannot run more than 8..... the anker is the best
^ Looks like it has 2 DC input ports.
Quote
Additional: AC adapter US$10.00
Makes me think that you can use 2 5V/3A adapters to get 6A meaning it can run atleast 12 most likely 16.