hmm... Maybe the upper limit is zero - 1 is too many.
Some random questions:
Seriously - have you ordered other iPads recently? Maybe you have exceeded some arbitrary number that you have ordered within the last month, or within the last year (?) Any possibilities, or no recent Apple orders?
Or, maybe some other person has ordered for your household (probably to your own address...?
And, this is where the inconvience comes from... You should call Apple customer service/AppleCare support who should be able to answer your question, or point you to the right number to call (might even be customer relations, I suppose)