How much r134a Freon?

This chart shows 40 ounces. This model should have come with R-12. Only put in R-124 if someone previously retrofitted it. If that has been done, you'll find the correct ports to fit the charging hose. The oil will have been replaced too to one that's compatible with R-134.

Also be aware there is no way on a GM model to know how much refrigerant is in the system except when you start with nothing and keep track of what you add. Chrysler used a sight glass on their receiver / driers. When the vapor bubbles were no longer visible. The system was fully charged. Ford used a sight glass too for a while, but they didn't work. There would still be vapor bubbles visible when the system was fully charged.

If you don't know how much refrigerant you started with in the system, you're better off not adding any more than what it takes to get satisfactory performance. If you over-fill it, liquid refrigerant can slosh into the compressor. At a minimum, which will lock it up. That can also damage it. When you have too little refrigerant in the system, that lowers the pressure, then more of the liquid will vaporize, expand, and the pressure will go right back up to normal. If you bleed off still more refrigerant, the same thing keeps happening. The pressure drops allowing more liquid to vaporize and expand, and the pressure goes back up. That keeps on happening as long as there's a little liquid left in the system to vaporize and expand. That makes reading static pressures very ineffective. Some people will tell you they can determine that state of charge by watching the high and low gauges with the system running, but I've seen multiple people interpret those observations differently on the same vehicles. Ambient temperature and amount of air flow affect those readings too.

Conversely, if you add too much refrigerant, some of the vapor will condense back to a liquid. That takes up less space, and the pressure goes back down to normal. This also repeats as long as there's some vapor in the system.

Home refrigerators are extremely intolerant of an over-charge of as little as two ounces. Automotive systems are designed to be more forgiving because of the hills, bumps, and turns the systems go through. The proper amount of refrigerant will result in the evaporator being filled halfway with liquid and the upper half with vapor. Where they meet is where the refrigerant is evaporating and where it is becoming very cold. With an over-charged system, that cooling could occur in the hose leaving the dash. You don't want the cooling to occur under the hood. Likewise, with too little charge, the evaporation will take place in the hose coming into the evaporator. Again, the cooling will occur under the hood instead of inside the passenger compartment. When temperature sensors or bulbs are used, they're positioned in specific places on the evaporator. When the cooling is occurring in the wrong place, due to an over or under-charge, those sensing devices detect the wrong temperatures resulting in improper system performance. The entire system could have not a single defect, yet performance won't be satisfactory from a simple incorrect amount of refrigerant. The only way I know to remedy that is to recover all the refrigerant, then start over with a measured amount.

For your viewing pleasure, here's links to some related articles you might find of interest:

https://www.2carpros.com/articles/how-a-car-air-conditioner-works

https://www.2carpros.com/articles/re-charge-an-air-conditioner-system

https://www.2carpros.com/articles/air-conditioner-how-to-add-freon

Please let me know how this turns out or if you have other questions.