Yes you can surely give improvement exam for any one subject or nore than one depending upon your satisfaction level.
However for admission into JEE mains, your aggregate score should be nore than 75% across PCM category.
You can also be in top20 percentile in Haryana board.
For SC and ST category, this criteria is relaxed to 65%.
So if you meet above criteria you dont need to appear for improvement exams and simply apply using jee2018 scores.
Else youcan reappaear in 2019 for JEE Main.
https://engineering.careers360.com/articles/jee-main-eligibility-criteria

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