Let me see if I can clear some of this up for you.
The stoichiometric ratio (AFR) for the petrol is 14.7 : 1 right?
This is correct, and it's important to understand the concept of stoichiometric ratio. It means that when the fuel and air are burned, they must be in this ratio if you want them to combine completely, with nothing left over (well, none of the original compounds).
So it is, If 1ML of petrol is used,then 14.7ML of air is used, right?
No, this is not correct. The ratio is concerned with the amount of compounds on a molecular level. Since the liquid fuel is much, much more dense than air, you can't use a measurement of volume like milliliter (ml). If you want to imagine the quantities side by side, you have to atomize the fuel into a gas, then combine the 1ml of fuel-gas with the 14.7ml of air. Or you could weigh the fuel and air and combine them.
So, if I have given the throttle more and more, the air part will be reduced much and much?
The Wikipedia article has a good explanation of the air-fuel ratio and how it changes:
A stoichiometric mixture unfortunately burns very hot and can damage engine components if the engine is placed under high load at this fuel–air mixture. Due to the high temperatures at this mixture, detonation of the fuel–air mix shortly after maximum cylinder pressure is possible under high load (referred to as knocking or pinging). Detonation can cause serious engine damage as the uncontrolled burning of the fuel air mix can create very high pressures in the cylinder. As a consequence, stoichiometric mixtures are only used under light load conditions. For acceleration and high load conditions, a richer mixture (lower air–fuel ratio) is used to produce cooler combustion products and thereby prevent detonation and overheating of the cylinder head.
So, the answer to your question is, yes the air part of the ratio is reduced. You could also say that the fuel is increased, but the result is a ratio with too much fuel. The extra fuel is not burned, but has a needed cooling effect.
In case I misunderstood your statement, when you step on the throttle the overall amount of air increases. The throttle is basically an air control - more throttle, more air (and more fuel). The ratio is decreased as explained above, but to overall quantities of each go up.
Up to which point this air can be reduced?
I could use some help on these numbers, but I believe that an AFR range of 12:1 (rich) - 16:1 (lean) can be found in an engine.
When the mixture is way to rich or lean, getting the mixture to ignite will be a problem, and even if combustion happens, it won't be very powerful.
So, in the Carburetor, the butterfly valve will do this and in the FI, ECU will determine this one right?
In a carbureted engine, the carburetor is responsible for mixing the right amount of fuel with air. The butterfly valve allows more air in, and more air flowing through the carb causes it to use more fuel.
In a car with EFI (Electronic Fuel Injection), the computer (ECU) uses an air flow meter (MAF) to determine the amount of air going in to the engine, and then calculates how long each fuel injector should be fired to give the correct ratio. There is still a butterfly valve that controls the amount of air.