My air conditioner needs freon how many do i need to put in it

Freon does not evaporate nor dissipate. If you're low on freon there's a leak. If it is low by 50% or more you will end up with air pockets in the system & air will keep the system from cooling correctly & dramatically shorten the life of the compressor. A local mechanic will check the system for leaks, remove the existing freon, examine the fittings, expansion valves & seals, replacing worn ones, **** the system down & recharge it...the going price for this service is $ 150.00 compared to $ 1000.00 up to replace the unit, if you make a mistake trying it yourself.