i have asked about board exam not mains

Okay, if you fail in more than one subject then you need to reappear and try to clear the exams in second attempt. You can reappear 3rd time also, but as a private candidate only. So, put in all your labour and try to clear it in third attempt. You can keep reappearing until you clear all subjects. For that refer to the official site and consult your teachers. Apart from boards, you have other career options too. Feel free to ask if you have further queries!