There are _______ rational numbers between two rational numbers. 

Solution: 

Let us explain this with an example. 

Let us take two rational numbers $$\frac{-5}{4} and \frac{5}{4}.$$ 

Rational numbers between them are $$\frac{-4}{4},\frac{-3}{4},\frac{-2}{4},\frac{-1}{4},\dots \dots \dots \dots ..$$ 

This proceeds infinitely. 

Hence, there are infinite rational numbers between two rational numbers.