how much questions we actually need to attend to score 99+ in CAT

A minimum of 60 questions is must for attempt and making atleast 40 correct answers is the key to 99 percentile as per CAT 2012 scorecard analysis. Your total score should be 300+ arohnd for aiming the 99 percentile. However, this varies as the percentile is derived from normalization process where you need to correct answers in lesser time. This mastery will come when you solve past5 years CAT papers and give atleast more 10 mock exams. With practice comes confidence in selecting right answers with 100% probability and success rate. Hope it helped your doubt get clear.

It's difficult to predict a number, as it varies every year.

It is not the number of questions that would determine but the accuracy in attempting the number of questions. You should ensure that whatever u attempt should be correct and try to maximize the score. You cannot say as to how much you need to attempt for this much percentile.