You can use a single Apple ID with a maximum of ten devices and computers (combined). Only five of them can be computers. So if you're only authorizing iPads and no computers, then you can authorize 10 iPads with one Apple ID.
Here's some verbiage from Apple's Terms and Conditions:
Association of Associated Devices is subject to the following terms:
(i) You may auto-download iTunes Auto-Delivery Content or download previously-purchased iTunes Eligible Content from an Account on up to 10 Associated Devices, provided no more than 5 are iTunes-authorized computers.
(ii) An Associated Device can be associated with only one Account at any given time.
(iii) You may switch an Associated Device to a different Account only once every 90 days.
(iv) You may download previously-purchased free content onto an unlimited number of devices while it is free on the iTunes Service, but on no more than 5 iTunes-authorized computers.