You would just need to have bulk account status, which means spending or crediting your account with $500 in the past year. More information can be found here: bulk prices
There is no limit to the number of domains you can order in bulk.
[This post has been edited by dynadot_staff on Jul 31, 2007 12:03pm.]
When you log into your account the first page you see is the summary page. On this page will be the exact amount you have spent over the last 12 months. Deduct this amount from $500 and this is the amount you will need to prepay (using PayPal is fine) into your account to get bulk pricing.