What % is considered of class 12 is it % attained in best four subjects or overall % attained in class 12

My overall % in class 12 is 69%.and %.in PCM is 61%

So you have fulfilled the second criteria but not the first one.Don't worry because if you do not have 75%, then you will not be able to apply for IIT's, NIT's or IIIT's but you can apply for colleges other than them.If you are desperate for them, then you have to give Improvement Exam next year.All the Best!!!

In how many shbjects i can give improvement

Pritanshu, it depends. If you want iit, nit or iiit, you have to give improvement exam of all five subjects.If not, then you can give improvement exam of 1,2,3 or how many you want to give for.Improvement exam will be held next year and once you give it, you will have TWO marksheets of class 12th. You will have to carry BOTH marksheets when you go for a job interview.The application form for improvement exam is likely to be uploaded on http://cbse.nic.in/ in September but keep tracking from August.Feel free to ask more.All the Best!!!

If i give improvement exam in one or two subject and my % becomes 75% or  more ,then can i be eligible for iit's and nit's

As i have already said, that you will have to give improvement of all five subjects to be eligible fot iit, nit and iiit.But, do not take so much stress because you just have to perform well in a couple of subjects and your percentage will surely surpass 75, you just have to get a respectable score in other three subjects.In improvement exam marksheet, only those subjects will be updated in which your marks increase from previous year.Hope I have cleared your doubt.