Hello aspirant, 

It all depends, like if you do not have 75% marks 12th,then in order for being eligible for admission through IIT JEE, you have to take the improvement exam. So, if you have score more than 75%, then there is no need for giving improvement in all the 5 subjects,as you are already eligible for JEE exam.However, a student needs to give improvement exam in all five subjects to be eligible for JEE. They are not allowed to appear in additional subjects and the subjects, in which they are marked as failed.

Hope your query is cleared. Feel free to ask more questions.