The Onename webapp is not meant for bulk registrations of usernames. The underlying system, Blockstack, has a registration fee and currently the Onename webapp foots the bill for it.
If you plan to register a lot of usernames through the Onename webapp, your registrations might get flagged as spam. See this article for more details.
People who want to register more than a couple of names should use the Blockstack software directly and pay for the names directly. More details on http://blockstack.org.