Pull the plug & if some runs out - you got enough. If none runs out, add till it does dribble out.
Careful though, there are 2 plugs. One (the lower one) is for draining the tranny fluid and the other (higher one) is for adding/checking fluid level.
But turn the vehicle on in nuetral with the brake. then release the upper pan bolt. It will dump a whole quart if the engine is off. If nothing comes out then pump tran fluid in with a hand pump.