There should be a data plate or sticker under the hood with the proper quantity listed on it. Some owners manuals have the same info listed as well. If all the freon has leaked out then you probably need a professional service to find and fix the leak before putting in the new freon. It may need to have a vacuum pulled on it to prepare it for a full freon charge. Trying to put a full quantity of freon into a partially charged system can blow out seals. If it is an older car that ran on R-12 freon then you should have the system flushed, and change out the expansion valve and receiver dryer before converting it to an R-134 charge. The lubricating oil from the R-12 is incompatible with the oil from R-134.
