So when I registered for jee main exam did I register it for only one exam in January or is it for both the exams in the year.

When you applied 3-4 months ago for JEE main then you applied for JEE main January session only and not for the second exam that is JEE main April session if you  want to appear for JEE main April session then you need to again apply for that the application for form same will begin from February 8 onwards there for if you are interested then you need to again apply by paying the required fee and then you can appear for the exam if you appear for board exams then best of two scores will be counted for the rank calculation and if you appear for only one exam then that its score will be counted for the rank calculation.

JEE Main Entrance Exam will be conducted twice a year ( January & April), The candidate can choose to appear in both the attempts. However, the attempt for a particular academic year will be considered as One.

It's your choice Whether you wish to give one attempt or two is entirely up to you. In case you have attempted both the exams, the better of the two scores will be used for the purpose of ranking in the merit list.

In case you have any other query then let me know.

Good Luck!