The Generalized Pigeonhole Principle

 If N objects are placed into K boxes , then there is at least one box containing at least 

$$\lceil \frac{N}{K}\rceil$$ objects .

( Notice that$$\lceil r\rceil$$ is the ceiling of r )

Example: 

Among 100 people , there are at least $$\lceil \frac{100}{12}\rceil =9$$ who were born in the same month .