Shmuel,
Open the hood of your car. Look on the underside of the hood or where the hood meets the car and you should find a sticker that looks like this. This is the capacity of your system.
But, as previously said on this thread, you can't just blindly add refrigerant. Too little is not good and too much is not good.
If it kind of works, but is weak, you might be able to add a little and see how it changes the performance. That is best done with a set of gauges made for air conditioning work.
If you system is not working at all, you probably have a leak and someone needs to find the leak and fix it first. Then it needs to be professional evacuated and leak tested and the refilled. If you have an accident or otherwise had to change a hose in the air conditioning system, then is has been open to the atmosphere and it needs to be professional evacuated and leak tested and the refilled.