Strangely, Kickstarter deliberately restrict buyers to just one pledge.
You can get around it by signing up for a new Kickstarter account, possibly using a different email or username, for each additional pledge you want to make. I would prefer this method as it makes it easier for me to keep track of orders.
Alternatively you may be able to increase the amount you pledge for a poster, and email me details of any others you want to buy. If the print has sold out I'll notify you when I run my daily stock check.