Where does IDFC says that 506250 million revenue will be on per month basis? It says " Rs 5,06,250 million in revenues and ARPU of Rs 300 per month after three years" , which in my view means that in three years it will be able to get this much revenue.
So number of Users required with ARPU 300 per month will be 1687 divided by 36 , that's around 46-47 million Users.
If we consider revenue figure on yearly basis (industry norm), the number of subscriber shall be around 140 million to reach mentioned figure.
I missed the 300 per "month" part