Explanation:

- Nature of litigation: The type of legal case can influence the number of witnesses needed.

- Number of issues required to be produced: More issues may necessitate more witnesses to address each point.

- Nature of the issues: Complex or technical issues might require experts or additional witnesses.

- The fact as to on whom the onus has been laid: Understanding who needs to prove what can determine the number of required witnesses.

- The specified purpose for which a particular witness is required to be produced: Each witness might serve a different function, such as expertise or personal account.

- Option 1 includes points I, II, IV, leaving out crucial consideration of complex issues and specific witness purposes.

- Option 2 misses the evaluation of litigation nature and the number of issues.

- Option 3 lacks consideration of the case's nature and specific witness roles.

- Option 4 includes all necessary considerations and is comprehensive.