Section 141(3)(g) of the Companies Act, 2013 deals with this. As a thumbrule you "An auditor can't accept more than 20 audits." Section 141(3)(g) prescribes the that a person shall not be eligible for appointment as an auditor if he is in full time employment elsewhere or a person or a partner of a firm holding appointment as its auditor, if such person or partner is at the date of such appointment or reappointment holding appointment as auditor of more than twenty companies other than one person companies, dormant companies, small companies and private companies having paid-up share capital less than 100 crore.

In the case of a firm of auditors, this limit is 20 company audits per person. In the case of an audit firm having 2 partners, the overall ceiling will be 2 × 20 = 40 company audits.