mene 2020 me 12th ki exam di he to me ab 12th bad kitni bar jee main ki exam de sakta hu

As per the eligibility criteria of Jee Main, a student can appear for Jee Main in three consecutive years starting from the year of passing his/her class 12th or equivalent exam. Now you have passed your 10+2 in 2020 so you can appear for Jee Main in three consecutive years and those are 2020, 2021 and 2022. Also Jee Main is conducted twice a year - One in January and another session in April. So each candidate has the chance to appear for Jee Main two times for three consecutive years which means one can sit for Jee Main for maximum 6 times - January and September in 2020, January and April of 2021 and  2022. And it's not mandatory to sit for both the attempts, if you appear for both the Sessions of Jee Main in a year, the best of the two scores will be considered for admission. You can check the below link for furthur details http://engineering.careers360.com/articles/jee-main-eligibility-criteria