Number Needed to Treat (NNT) is an important epidemiological calculation that shows how many people have to be treated to preventing a future bad outcome. In a perfect situation, the NNT would be 1, meaning that every person that you treat will not get the bad outcome in the future.

NNT is calculated using other epidemiological calculations:

Control Event Rate (CER) - Proportion of the control group with the outcome
Experimental Event Rate (EER) - Proportion of the treatment group with the outcome
|CER - EER| is the Absolute Risk Reduction (ARR) or the effect of the treatment in comparison to a control treatment.
Then, in order to calculate NNT, just use (1/ARR).  

With smaller values of ARR, or small treatment effects, the NNT will increase meaning that more people have to be treated to prevent a future bad outcome. Higher values of ARR show a strong treatment effect, fewer people will have to be treated to prevent a bad outcome because it is so efficacious.