How many atoms are there in a single molecule of ammonia?

The molecular formula for ammonia is ${\text{NH}}_{3}$. In a molecular formula, the subscript for each element indicates the number of atoms of each element. When there is no subscript written, it is understood to be 1.
In one molecule of ${\text{NH}}_{3}$, there is 1 atom of $\text{N}$ and 3 atoms of $\text{H}$, for a total of 4 atoms.