Hii aspirant..

The NDA Exam is conducted by UPSC twice in a year - April and September.

Eligibility for exam is-

Nationality: A candidate must either be (i) a citizen of India, or ii) a subject of Bhutan, or (iii) a subject of Nepal, or (iv) a Tibetan refugee who came over to India before the 01st January 1962 with the intention of permanently settling in India, or (v) a person of Indian origin who has migrated from Pakistan, Burma, Sri Lanka and East African Countries of Kenya, Uganda, the United Republic of Tanzania, Zambia, Malawi, Zaire and Ethiopia or Vietnam with the intention of permanently settling in India

Provided that a candidate belonging to categories (ii), (iii), (iv) and (v) above shall be a person in whose favour a certificate of eligibility has been issued by the Govt. of India
Certificate of eligibility will not, however, be necessary in the case of candidates who are Gorkha subjects of Nepal.

Age Limits, Sex and Marital Status: Only unmarried male candidates born not earlier than 02nd July 2001 and not later than 01st July 2004 are eligible

See NDA Educational Qualifications

(i) For Army Wing of National Defence Academy: 12th Class pass of the 10+2 pattern of School Education or equivalent examination conducted by a State Education Board or a University

(ii) For Air Force and Naval Wings of National Defence Academy and for the 10+2 Cadet Entry Scheme at the Indian Naval Academy: 12th Class pass of the 10+2 pattern of School Education or equivalent with Physics and Mathematics conducted by a State Education Board or a University

Candidates who are appearing in the 12th Class under the 10+2 pattern of School Education or equivalent examination can also apply for this examination

Candidates who are appearing in the 12th Class under the 10+2 pattern of School Education or equivalent examination can also apply for this examination.

There is no restriction on number of attempts for the exam , the only condition is that you fulfill the above mentioned eligibility criteria.

Hope it helps.