HI, i got 86.6% in cbse 12th exam and i scored only 68 in maths. can i go for rechecking to increase my marks? or will it decrese my marks?

You should definitely apply for reevaluation if you are confident that you were supposed to score better. There are various steps in the procedure. When I applied for reevaluation, there were three steps: first, they check for calculation mistakes, if any, in sum total. Then they provide you a copy of your evaluated answersheet so that you can easily check where your marks should be increased and thirdly, you are asked to apply for reevaluation of those questions where you think you should be getting more marks with proper reason and reference books link,  etc.

So, there is no chance of marks deduction till 2nd step. In 3rd level, there are chances, but if you are confident, GO FOR IT!

I got 1 mark increased in English paper after reevaluation. But there are students who have got many marks increased.

The procedure costed me around Rs.2000 and there was some refunding too, I guess, for the questions that were previously incorrectly marked. It took few weeks in the procedure.

In all, you can decide up to 2nd level whether you wish to go for further reevaluation or not.

Also, check the cbse board official site for knowing the procedure, since I have answered on the basis of 2016 board exams.

Hope this helped. All the best!