If i scored 120 in jee mains 2018... What could be my rank..???

NTA Scores are given in percentile.
Percentile is calculated by the ratio of the number of values to the total number of values . You cant say/depict your actual score/rank from your percentile neither can you calculate it by only marks. To calculate the actual score/rank/percentile you would need to know the number of students who appeared in the examination and the highest score, average cut offs and the highest score. while as marks are calculated based on the right questions you have attempted, out of the total marks you have obtained for the right number of questions marks from wrong questions marked are deducted from the total.

NTA announces the result of JEE Main/Advance in the form of a percentile score in every attempt or exam of JEE Main/Advance. The All India Rank (AIR) of candidates in JEE Main is calculated by taking the best score out of four attempts and are provided after the result of fourth exam. Candidates are able to check the detailed marks vs ranks of JEE Main/Advance after the conclusion of the exam.

Two types of ranks are given at the end:

1. AIR: All India Rank
2. CR: Category Rank

AIR is the overall rank obtained by the candidate respective of their category and CR is the rank candidates got in their respective category (General/EWS/SC/ST).