Here's what it says on the Ticketmaster site under Ticket Limits:
There is a 2 ticket limit for all password-protected presales and membership bundles. There is a 4 ticket limit during the first day of the onsale.
Please adhere to published ticket limits. Persons who exceed the ticket limit may have any or all of their orders and tickets cancelled without notice by Ticketmaster in its discretion. This includes orders associated with the same name, e-mail address, billing address, credit card number or other information.