Mr. Kulkarni
It varies from one municipal corporation to other, as per Pune Municipal Corporation total water requirement calculated as 135 ltrs per person per day.
Out of which 90 ltrs for domestic & 45 ltrs for flushing & other use.
Irrespective of flat configuration ( ie. 1, 2 or 3 BHK) it is assumed that 5 persons stay in a flat.
So for a Building of 10 flats of any configuration
10 flats x 5 persons x 135 ltrs = 6,750 Ltrs per day is the water requirement per day.
The UGWT is usually kept 50% more ie. 6750 x 1.5 = 10,125 Ltrs ( excluding the free board of water)
for fire fighting refer NBC which specifies different fire fighting requirements as per the building use and size.
Thanks.