Just tried doing the same with the iPad and it seems they have stopped that as well. You can only do 10 per device as it seems apple checks the serial number upon enrolment.
This is pathetic if your trying to deploy them in a corporate environment.
Confirmed this with Apple Support, to which they basically said to me, go away and do it on each individual iPad.
I found this discussion because I need to create dozens of Apple IDs for the faculty and staff of the school for which I'm the IT Director.
This is weak.
Apple, PLEASE upgrade the Apple ID system so it represents the current needs and paradigm.
What am I thinking, Apple (like Google) don't follow user discussion forums, except by accident when an employee sees something and is motivated enough to carry the water for users in need.