What kind of freon do I add to a portable ac unit?

Actually, Freon is a trade name for Dupont. The correct term is refrigerant. You will need to look at the name plate on the unit. Depending on the age, it could be R-22, R-134A, or R-410A. It should also list the correct amount for the unit. Unless you are a qualified HVAC tech. with the proper equipment to do the work, I would advise you not to atempt to do this because those unit do not have service valves preinstalled on them. If it needs refrigerant it has a leak and needs to be repaired and the correct amount of refrigerant put back in.