Step 1: The answer depends on the terrain-based population norms set for Primary Health Centres in India.
Step 2: Standard guidelines fix one PHC per 30000 population in plain regions, but in hilly, tribal and difficult-to-reach regions the figure is lowered to 20000 to keep services accessible across a dispersed population.
Step 3: Since the question specifies a hilly area, the applicable coverage is 20000.
Step 4: The other figures are wrong: 30000 is the plain-area PHC norm, while 10000 and 50000 do not match any PHC standard, so 20000 is selected.
\[\boxed{20000}\]