Hi neeti2711,
The scoring algorithm on the Official GMAT is far more complicated than most people realize. It takes into account a number of different factors - far more than just the number of correct and incorrect answers - so you can't really think about your score in terms of "how many can I get wrong." Instead, you should try approaching that score goal from the 'bottom-up.' After reviewing your last CAT, how many questions did you get wrong because of a silly/little mistakes? How many of those questions were actually 'gettable?' THOSE questions represent missed points - so eliminating those mistakes from your work is a must to raise your score to the next level.
You reported a couple of 660s on two practice Tests back in August. How have you been scoring on your CATs since then?
GMAT assassins aren't born, they're made,
Rich