How much it takes, or how much it has in the system at any given time? To know how much refrigerant goes into the system, there'll be a sticker from the manufacturer - usually on the section of the HVAC compartment which extends under the hood. If this is faded or absent, the owner's manual should tell you. If that's unavailable, there are online resources.
As for what's in your system at the moment, the only way to tell is to evacuate, recover, and weigh the refrigerant you have in your system.
Usually, on most cars, this means that the weather is cool or that there is insufficient refrigerant in the system. It can also be caused by a restriction in the AC system or high temperature or excessive amount of refrigerant in the system.
R134a
The refrigerant itself does.
R134 a
It means that you have a refrigerant leak, and will eventually lose all of your refrigerant. Then your AC won't offer any sufficient cooling, and if you run your AC with an inadequate amount of refrigerant, you'll eventually destroy your AC system. So it means that, if you don't want this to happen, you need to get the leak located and fixed. The presence of air in the AC system indicates a likelihood of a leak (or the leak) being on the low pressure (vacuum) side.
FYI most refrigerants already have oil that is proportionate to the amount of refrigerant in the can. If you are using straight refrigerant and all you need is to add oil to your system, then there is product out there that is straight oil that can be added to the low side port in your ac system. IDQ , QUEST , and AC PRO are some companies out there that have many of these products. You can also find them in the web at www.idqusa.com along with helpfull information about your vehicles ac system.
The refrigerant itself does.
Generally you charge a refrigeration system by weight, not by pressure or temperature. Say, for example, if you were adding refrigerant to a system, you might add a pound of refrigerant. You wouldn't add it until it was at a certain pressure. As for a consumer AC system in a vehicle, I couldn't tell you how much to add. Look for a reference of the amount of refrigerant in the system and start by adding half that amount maybe?
It compresses the refrigerant (probably r134a) and pushes it through the ac system.
One possibility is excessive refrigerant in the system.
Can be added with refrigerant check can contents
R-134