what are the dimensions of the bucket...with my ebb and flow system I would flood it 7 times a day for 15 minutes each time...i started with six added one more and didnt see any problems with it. I converted my ebb and flow system to a bubbleponic system so I dont have to worry about the flood problem anymore. I was also running 24/0 ratio light:darkness. if you were to do 18/6 id do 4-5 floods a day. Remember that you dont need to flood it during the darkness cycle, only during the light cycle. and that within an hour of the light cycle starting it's going to need a watering.
but if you gimme the dimensions I'll give you an estimate of how many plants to put. Also look into Sea of Green technique, or SOG technique. This is a technique of putting many plants in a small area and with research you could prolly figure how many to put in there yourself.