The number you need varies... Most things I have seen say anywhere from 10-50 pounds per acre. It all depends on your goals with the fish. Forage, FA control, aquatic vegetation control, etc...
Best time of year to stock is when you can safely bet water temps will stay above 60 degrees. Tilapia really won't start spawning or growing much until water temps hit 70 and won't really hit full speed ahead until water temps are in the 80's...
Since tilapia are sold in pounds and not by number, you usually are sold mature adult fish in the 6"-9"+ range. From the sounds of your pond holding almost not bass over 12", tilapia in that size range should survive and not be predated upon. However, their babies will be candy to those hungry mouths. This should take the pressure off your BG when they spawn. Your original stocker tilapia will eat the FA, clean up other organic build up and if your are interested, you can fish for them and keep a few for the table as well.
George1, Rainman among others would probably say they control FA rather well... As with almost anything, it all depends.
