The rule of thumb is two to four pounds of refrigerant per ton of your air conditioning unit. Now most home air conditioning units are between one ton and five tons. Anything over five tons or greater is considered a commercial unit.
Let’s say you have a two ton home unit. If we do the math of four pounds of refrigerant times two tons of your unit you end up with eight pounds of refrigerant required.
As I said above that is the typical rule of thumb. Each machine/unit is different and each manufacturer is different. It is always best practice to physical check your unit and see exactly how much refrigerant is needed.