These vary a bit depending on the lab, so the ranges that follow are guidelines rather than absolutes. Be sure to ask for a copy of the CBC each time it's performed, and look at the lab results sheet for the normal range for that lab. Still, there are some general rules of thumb:

Red blood cells (RBC)
Normal for men: 4.5 to 6.2 million per microliter (a single drop)
Normal for women: 4.2 to 5.4 million per microliter

White blood cells
Normal for men and women: 3,700 to 10,000 per microliter
Lowest level at which patients are safe from infection: 1,000

Platelets
Normal range for men and women: 150,000 to 450,000 per microliter
Lowest level at which blood can still clot normally: 100,000
Level at which there's a risk of spontaneous bleeding: 50,000
Level at which bleeding can become life-threatening: 5,000