The oxidation number of nitrogen in ammonia ( NH_3 ) is:

Let the oxidation number of nitrogen in $\text{NH}_3$ be $x$.

Hydrogen is bonded to a more electronegative element, so its oxidation state is $+1$.

The sum of oxidation numbers in a neutral molecule is zero.

$x + 3(+1) = 0$

$x + 3 = 0$

$x = -3$

The oxidation number of nitrogen in ammonia is $-3$.

Answer: $-3$