As far as I see, there is a formula to calculate the fuel consumption from Manifold Air Pressure reading, but some of the cars has no MAP sensor. Is there a formula to calculate the fuel consumption from Intake manifold air pressure for example ?

I think you might be getting mixed up between manifold air pressure and mass air flow.
Mass air flow gives you the amount of air going into the engine. Manifold pressure is just how hard that air is being pushed. The amount of air has to be mixed with an amount of fuel to get a good ratio for combustion and the right amount to drive the piston.

Mass airflow sensor is one of the PIDs that is part of OBD2. In Canada and the USA, these are legally required. Mass airflow sensor is Mode 01 PID 66 (hex). This value needs to be calibrated; the maximum value is Mode 01 PID 50 (hex).

The cars I’m testing on can give the Intake manifold air pressure, but not the mass air flow; As I can see, there is a way to calculate the MAF, but I can’t find a formula that it’s clear enough.