As per the Companies Act, 2013 and the rules thereunder, the following companies are required to appoint an internal auditor:

1. Companies with a paid-up share capital of Rs. 50 crore or more during the preceding financial year;
2. Companies with an annual turnover of Rs. 200 crore or more during the preceding financial year;
3. Holding companies and subsidiaries of companies that meet the above criteria;
4. Companies that are required to get their financial statements audited under any other law, such as banks, insurance companies, and certain classes of companies engaged in production or generation of electricity.

The internal auditor appointed by the company must be either a Chartered Accountant or a Cost Accountant or such other professional as may be decided by the Board. The internal auditor is responsible for examining the adequacy and effectiveness of the company's internal control systems, risk management processes, and compliance with applicable laws and regulations. The internal auditor reports to the Audit Committee of the Board.

It is important to note that even if a company is not mandatorily required to appoint an internal auditor, it may choose to appoint one voluntarily for better internal control and risk management.