Any E-mail would work
So just use multiple e-mails accounts if you want to buy many.
The deal renews itself when it expires...I just rushed through a deadline and now it's another 23 hours...so technically the deal doesn't expire
If you don't want to create multiple email accounts AND you have Gmail - you can use the dot-trick [cybercashcows.com]
Don't buy more that 5 vouchers, unless you plan to use them across different Skype accounts. There is a redemption limit of 5 vouchers per unique Skype account.