How many attemps will I get in JEE if I give board exams again?

Your eligibility for appearing IIT will be taken into account only when you pass out your Board with the combination of Physics, Chemistry and Mathematics and your earlier passing with other combinations with a recognised board would not be considered as one of your first attempt.
Your combination of Physics, Chemistry and Mathematics would be treated as your maiden attempt and considering the same being your first attempt towards writing your IIT, you will get the privilege of writing two advanced JEE.
You should not have any ambiguity regarding your attempt for IIT.

As per the stipulated rules and regulations of JEE advanced examination, a candidate will be given two chances. The first attempt should be in the year in which you cleared your 12th examination. You will be given one more chance the next year. By any chance if you are failing to attend the JEE in the year you have passed you will be given only one chance.
you want to do the 12th class from a different board again with a different subject combination. After passing this if you want to appear for JEE advanced with this new 12th certificate you can attend 2 times provided you are not attending this year with the 12th certificate you obtained this year.
If you want to attend with the present certificate you have to attend this year.

At first, for appearing in IIT exam you should have passed class 12 th board exam with a combination of physics, chemistry and mathematics. Candidates can appear for JEE main exam six times over three consecutive years. In a year, a candidate can appear for JEE Main exam twice in the month of January and April. Suppose If the candidate has passed the 12th exam in 2021then the candidate can appear in 2021,2022and 2023 JEE exams. If the candidate is appearing for the class 12th board exam in 2020, then he/she can appear in both the sessions of JEE main in the years 2020, 2021 and 2022.

The first chance for attempting JEE is the year in which you pass out your class XII. After that next year, you get a second chance. Now you are going for a repeat class XII exam naturally for improvement in your score. So instead of attempting for JEE against your earlier class XII exam, you can do it against the second class XII exam and that would be counted as the first attempt for you. I am not able to understand what benefit you would derive by doing that because your performance in JEE has nothing to do directly with your class XII exam results and maybe you fare quite well in JEE.
From your post, it seems that you have missed the JEE so far and want to attempt it after doing class XII again. You can do that and if you follow the rules then you can avail yourself of two chances for it mentioning your second class XII exam as the basis for it. I do not see any problem on that account.