If You really Ask me, then let me tell you.
The number of cement bags per cum of conrete depends on Your Concrete Mix Design.
Mix Designing is done to achieve a specific characteristic of Concrete like compressive strength, durability etc.
Characteristic of concrete depends on design of structure, life span etc.
So one can not say how many bags of 50kg cement will be required to produce a cum of concrete without knowing a what mix it is.
Even if in Todays world, you are doing concrete in conventional way then you are still in stone age.