Actually, you can't have more than 3 accounts tied to the same credit card in iTunes. And Apple support (online, chat, email, phone, store) is no help in figuring out what is going on. When you try to setup the 3rd account with the same credit card, iTunes says "Contact iTunes Support to complete this transaction" but does not provide any contact info. I've been trying to set up the 3rd of 9 accounts I need to setup for our corporate iPads for 2 days now. Support did some digging and after an hour on the phone, told me I could "have each user just set up their own iTunes account with their own credit card".
This is so remeniscent of steps another computer company stumbled over when they started to sell to Corporate America. Apple does not get the corporation. Product looks good but there is a lot of unfinished business when it comes to administering multiple devices. Support is friendly but not very able to solve the problem.