Again its a old question, not revant form Co ACt 2013 Point of View but below is suggested answer with fe assumptions
 
As per Sec 141 (3) No Company or its Board of Directors shall appoint or re-appoint any person or Firm as its Auditors if –
(a) Such person is in full time employment elsewhere or
(b) Such person or Firm holds the office of Auditor of the specified number of Companies or more than the specified number of Companies.
2. Ceiling Limit: The ceiling limit is 20 Company Audits per person. However, the following audits are excluded in while computing ceiling limits-

OPC
SMALL Company
Dormant Company
Private company having paid up capital less than 100 crores

Branch Audits (As they are not equivalent to company audit as per old department clarification)
Foreign Company (Because they are not covered in definition of company they are body corporates for indian law and not registered company in india)
3.Analysis: In view of the above, the position of the Firm is as under Eligible Number of Audits for (P, B & S at 20 each) = 60
Less: Existing Audits (excluding Branch Audit 60 -2) = 58
Balance that can be taken = 2

4.Conclusion: Of the three audits which are offered now, the ceiling limit is not applicable in respect of Private Company and the Foreign Company (in view of Point 2 above). Since the Company is eligible to undertake one more Company Audit (as per Point 3 above), the audit of the Public Company that is offered now, can be undertaken.

Assuming paid up capital is less than 100 crores for private company