I've done 8 so far, I think. I hoping to finish around 20+ exams. LOL I'm hoping to improve as well!
No offence, but doing this amount of practice exams is useless unless you are actually understanding and comprehending your mistakes. If it takes you 20 exams to get things "perfect" then that's fine. Remember that after every exam you should go over your common errors and revise the THEORY behind your mistakes. Otherwise, it is pointless doing this amount of exams.
However, if after 5 - 10 exams you are finding that you are only making silly errors, well then generally you're probably not going to get anything from overloading on exams. You're better off ensuring you make use of your reading time appropriately and understand the requirements of the question before you answer. Remember you want to ensure you keep fresh for the exam in November, its only the end of September right now.
Personally I found I did about 8 - 10 exams for the mid year, that's the amount I needed to get pretty much spot on. Then in the lead up to the exam (the week before) I did an exam every two days just to keep fresh.
Just my 2c and remember everyone is different so have differing exam preparation techniques.
MBA.