Hello,

UPSC is the authority which conducts the NDA Exam. NDA Exam is conducted in two sessions in a year. The one is in April, and again in the September. It is totally your choice, whether you give only NDA 1 or NDA 2 or both. But, to give the NDA Exam,  you have to fullfill the eligibility criteria. Below, I am providing a link. Here you can get all the stuff related to this exam.

https://competition.careers360.com/exams/nda

Hope it helps.