(i) In case of one person company, the requirement of directors is one.
(ii) In the case of other Private companies the minimum requirement of directors is two

Causal vacancy is a situation in which a seat in a delibrative assembly becomes vacant during that assembly's term, casual vacancies may arise through the death, resignation or disqualification of the sitting member or for other reason.

A person who is not the member of Board but has some power to run it can be appointed as the director but according to his/her wish.

 (i) Alternate directors are appointed by the Board of Directors, as a substitute to a director who may be absent from India, for a period which is not less than three months.
(ii) The appointment must be authorised either by the Articles of Association of the company or by a passing a resolution in the General Meeting.
(iii) The alternative director is not a representative or agent of Original Director.

(i) An Executive Director can be either a whole-time Director of the Company or a Managing Director.
(ii) But a Non-Executive Director is a Director who is neither a Whole-time Director nor a Managing Director.