Estimating a population for a water supply project can be difficult. It involves understanding some generally-accepted rules of thumb for population estimation, and more importantly, community-supplied information related to their cultural characteristics and economic circumstances.
A good place to start is with Arnalich’s Gravity Flow Water Supply handbook.
To take things further, and to better understand population estimation for water projects, I recommend reading pages 1 - 12 in Gupta’s Hydrology and Hydraulic Systems book. Link: https://drive.google.com/file/d/1jv_lkTjo1wSD8gVPfqHUKZua9U8gcLTE/view?usp=drivesdk