u r not understand my question. my question is- i hav passed 12th with pcb in 2021. i will again 12th maths as single subject in 2022. jee mains ke criteria ke according i have all subjects. can i eligible for jee mains in 2022

So basically you are saying that you have given 12th board with PCB this year(2021). And your question is  "Can I give only math next year and be eligible for JEE MAIN next year"?

See, first of all, neither the state board nor the central board gives you this facility to sit for one exam only and change your qualification from PCB to PCM.

But one board gives you this option and that is the NIOS board. Here you can sit for Mathematics paper only and after passing you can apply for JEE main. But it still doesn't help because at the end, let's suppose you qualify and going through procedures of admission in a particular college, they will ask you a single mark sheet where marks of all subjects should be given. Since you have given Mathematics from the NIOS board so you will have a separate mark sheet of it, but they will not two separate mark sheets into consideration and in the end, you will find that you wasted your time and energy.

But still, if you really want to get admission in some top engineering colleges then you have two alternatives.

1. Reappear for class 12th from any board but it will take your one year.
2. If you don't want to waste your 1 year, then reappear for all the subjects from the NIOS board, and not only for the Mathematics paper. In this way, you can get a single mark sheet which will contain all subjects' marks.