You're supposed to limit yourself to taking it less than twice a year. It doesn't increase your chances, but you should definitely not use it that frequently.
It's not good on your body to say the least. It's not meant for regular birth control. Taking it too often will mess your body up. It's really meant for those who went through failed birth control use such as -busted condom, forgot to take the pill, had an "uh oh"... you know? I don't recommend taking it that often. Just one plan b can make your cycle/body so wacky. As far as getting pregnant with taking it more than once in a month, I really don't know. But, I'm sure since one messes up your cycle, it could become difficult. I'm no doctor, but that's just my opinion.