Hi...

It is good that you have made a decision in your under graduation that you have to get into IIM.

You have to get a maximum percentage in your under graduation because the IIMs consider your under graduation marks for the admissions. As your marks increases then your ratings or the standard for your profile increases and you will have high chances of converting in IIMs.

You have already got a quite good score in your grade 10 and in grade12 you have cleared the minimum percentage this would be enough to get into IIMs.

But you have to do excellently in your CAT exam  your percentile should be above 99+ to get into top three IIMs. If you are doing your engineering in your under graduation then you have to do the best in your CAT exam.

There is never accurate number of calls from the IIMs.Every IIM has itsown criteria to call.the candidates and it may vary from one year to another. Other than percentile the other factors considered to bag a call from IIM are gender, your UG background, work experience. Just excel in your studies and do.your best in CAT which would fetch you an IIM.

Best luck