How many white blood cells should you have per cm−3 to keep bacteria under control?

Question

 

White blood cell counts. More than half the white blood cells in your blood are neutrophils (15µm diameter) whose job is to catch and destroy bacteria and fungi. They can creep along by amoeboid motion at a speed of about 1 body length per 20 seconds. Well-fed bacteria double their numbers every 20 minutes by binary fission. How many white blood cells should you have per cm−3 to keep bacteria under control? Assume the bacteria are at rest.