Just a good squirt. If you know you have a good coat of wax on, use a tad more. If you think most of your wax has faded, use a tad less. Just do a through rinse, and you will be fine.
For my car soap I'll use a little puddle of soap on slightly dirty cars and a big ole puddle for really dirty cars... Probably around 1 oz of car soap for 3-4 gallons on a lightly dirty car. 5oz+ for the really dirty cars.