The minimum number is 2!
Here is a solution which prints out 2.
Here is a solution which uses semaphores to protect access to the display.
Killing the balls: balls1.zip
Freezing the balls: balls2.zip
Freezing revisited: balls3.zip
A solution using
This solution is fair - the monitor methods are careful to change the direction of the traffic flow from time to time. Notice that fairness makes the solution much more complicated.