Hello Aspirant,  It is impossible to to say that what percentile you have when you secured 100 marks in the exam. 

100 percentile means that you have got highest score with high accuracy. For eg., if 5 students have got 200 marks then they all will have 100 percentile and after that the person with less marks.

So how many students in head of you it is not possible. 

I hope this will help you.