How much freon (R19) should be used to fill a home A/C unit. I need to replace the evaporator and must dischrage the system and refill it.

Only a licensed technician is allowed to dispense Freon, and you should find it difficult to purchase any Freon witout the proper credentials.

See EPA regulations:http://www.epa.gov/ozone/title6/608/

Also, Freon must be reclaimed by proper procedures, and not allowed to be intentionally dispensed into the atmosphere.

All you have to do to replace the evaporator, is pump down the freon into the outside condensing unit using the condensors own compressor. Do it all the time... No need to purge the system... Close off high side valve on the unit while running and wait for it to pump down to zero PSI and then shut down and close off the low side valve. Change coil and then pull a vacuum on the system. Release the charge from the condenser and check charge...