See for most of the boards there are no limits of attempts. But for some state boards there can be bounds that a student can only appear for 3 times in the examination. Now considering the board, passing it isn't that hard and you can easily crack it provided you are wiling to study and prepare well. Chemistry is a overall factual subject as well as logical, so you need to understand the logics as well as the information in order to be good at it.

You can follow the link below where the toppers have said about some preparation tips which can be really beneficial for you. Its not only for CBSE, but if you are from any board then if you apply the tips you can score good:

https://school.careers360.com/articles/cbse-toppers-preparation-tips-to-score-99-in-board-exams

I hope this answer helps. All the very best for your future endeavors!