In general cases maximum diameter of rain water pipe is 4'' because this dia pipe easly embeded in the 9'' thick wall during masonary. In case official or commercial building when length of building is more and non availabilty of 9'' or 13.5'' wall due to design. than rainwater is drained through duct with the help of 6'' dia pipe. Fix at least 6 nos 4'' dia RWP.
Generally 4" dia pvc pipe with 2-3mm thickness is sufficient for drainage of rain water.However calculations on catcment area,intensity of rain and other variable factors may be considered before construction.