Assuming the balls are thrown one at a time and that the minimum ball diameter is known then a "light curtain" is worth thinking about. Have LEDs on the rim of the bin shining on photoresistors. As a ball breaks the beam or beams you can count it.
The beams would need to be close enough together so that a ball cannot get through without breaking at least one.
You should look for a short change in resistance as opposed to a gradual change that might occur as ambient light levels vary. Before going too far it would be worth confirming that a fast moving ball causes enough of a change in resistance for accurate counting.
I agree, but instead of using regular LEDs which can give inconsistant readings when both the LED and ambient light go into the photoresistor, you should go with maybe infrared LEDs and IR receivers.(depending on budget)
OR
Use a single IR distance sensor or Ultrasonic distance sensor and set the sensor to read a certain distance like 5 inches to 1 inche (depending on hoop size). Set limiters so that it does not get set off when anything is greater than 5 inches or less than 1. When it detects something with in the set range, it will trigger you score board or whatever you want to use.