Hello Sneha Ji,
I agree with the replies of above members. In fact, your question itself gives its answer.
First be clear in one point that a CA can carry out maximum of 20 companies Audit in his individual capacity.
Secondly, a firm of CA’s can carry out Company audit to the tune of Number of partners * 20(individual limit). In no case individual limit should exceed.
Regarding the confusion in your question, remember that both Head office & its branches is one unit as long the appointment letter states that particular CA is eligible to carry out its audit. Suppose you are a CA of Reliance Digital equipments Ltd. The Pan India branches are 100 and you have got appointment letter (1st) to audit its Head office + 5 branches network. In this case your limit will be taken as audit for 1 company. Suppose you have left over limit of 19, you can very well use it for another assignment subject to the conditions of appointment letter & Sec. 224.
Now suppose if you get another appointment letter (2nd) from same head office regarding the audit of 1 more network at Delhi. This separate appointment letter will be considered as 1 audit in your individual capacity. So now you have exhausted 2 audit limits.
The above limit will be subjected to firm limit (if you are a partner in any firm say,PWC). Suppose PWC has exhausted its maximum limit of Company audit after your acceptance of 1st appointment letter & assuming you are not a partner in any other firm then you cannot accept 2nd appointment letter for PWC.
I hope i have made myself clear in the above points. Revert back, for any further doubts.