You'll be fine. USB allows for 127 devices per root hub, most computers have two or more root hubs. I suspect that's going to limit how many DAQs you can have on the system, not MAX.
I have one system with two panels of 7 DAQs per panel, both on their own hub.
Next project will double that (adding two more panels to that system).
The cheaper/slower DAQs (600x, 621x) do better on a hub than the bigger/faster ones (634x) do.
Ideally the DAQs are plugged directly into the computer, but that's not always possible.
Use a good powered industrial hub, not a cheap consumer-grade one. I like the Startech ones, i've had good luck with them:
http://www.startech.com/Cards-Adapters/USB-2/Hub/Mountable-4-Port-Rugged-Industrial-USB-Hub~ST4200US...
http://www.startech.com/Cards-Adapters/USB-2/Hub/Mountable-Rugged-Industrial-7-Port-USB-Hub~ST7200US...