It's not how much, but more of the "Volume" the gas you are replacing it with.
When you take on a Rock hit, that usually means the condensor unit - the outside one, that got hit, is compromised and that part of the system needs to be replaced.
So if you got that done, the rest is on the sticker on the front cowl.
The above is for a FORD Fiesta 2019 vintage...
I just have to caution you, because part of that "volume" be a lubricant to keep the bearings in the system working and not seizing up.
It (the lube oil) is supposed to be in a suspension in the can of refrigerant you use and is supposed to be compatible with the PREVIOUS refrigerant that was used in the OEM system. So that when it is dispensed out of that can into your system, it will have the proper ratio of oil to refrigerant - so shake the can well, and add some before you start the motor, to check for leaks - then there should be enough in the system that once the compressor starts - with won't lack fluid and lube.
Check with your own manual and always work with the instructions that come with your replacement can of refrigerant.
Then see if you can add the can into the system with the engine running with the blower and AC on (including that button getting pressed in at intervals - you may need a extra hand to punch that AC button on occasion and watch to see when the light will kick on), else they (your helper) will have to "trigger" the AC button as you add in the can while they see if the system can see the pressure change - increase - so the system can turn on that AC light. By then you should have used up most of that can (see sticker above) and you'll hear the compressor work once there is enough - but don't overfill it because it will shut down if you put too much in.
Why do it this way? Several reasons, you don't know what is left in the tubes of the heat exchangers.
You have two, rocks hit the external one. However, the internal one may have oil left in it, that will change the pressure of the system when you add in the can - as well as altitude and the outside temperature - can change the way the volume of pressure will work with the sensors to even tell the system to turn on. It's why you have to have the car running and the AC system on to force the sensors to tell the compressor to even kick in.
For Further Emphasis:
- I was recently working on one that had a similar "Fan won't kick on" issue that was not due to a fault as a leak in the system - but how the system thinks the Motor is ready to work with the AC unit. Was working with a 2016 Fiesta so this had a 1.6L NA motor (Sigma) and on a cold day he was having trouble with trying to clear the windshield of frost both inside and outside and thought the AC unit had failed.
- He tried putting in a 12oz can of AC R134a - we stopped the process in time before more damage was done.
- The Fan works as a means to take away heat from the outside condenser and pulls air thru the shroud to do this.
- If the Air temp sender is telling the PCM is too cold, that will not kick in the Fan but will kick in the Compressor as possible so you know. This is so the heat build up at the radiator and motor will allow the engine to warm up faster and keep hot as the AC works to clear and reduce the humidity to clear the windshield.
- It is by that process of "learning" the cold air pressures and the outside air temp on a cold motor can affect the warm up times - using the Fan as an indicator - requires you to let the car warm up first then use the Fan roaring noise as the indication the system is ok.
