What is the oxidation number of iron in $FeO$?

Hint :An atom's oxidation number in a molecule is determined by basic mathematical calculations based on its combining potential and/or valency. The calculations are completely dependent on the molecule's total charge.

Complete Step By Step Answer:
The inorganic compound $FeO$, also known as iron$\left( {II} \right)$oxide or ferrous oxide, is an inorganic compound with the formula $FeO$. Wustite is the mineral form of ferrous oxide. It's a black-colored powder made up of many iron oxides that are often mistaken with rust, which is made up of hydrated iron$\left( {III} \right)$oxide (ferric oxide).
Let us take a look at how to make calculations using the oxidation number. When we state that the calculations are dependent on the molecule's total charge, we can have one of the three possibilities:
$1)$ The charge on a neutral molecule is equal to zero.
$2)$ A Cationic molecule has a positive charge.
$3)$ An anionic molecule has a negative charge.
In a neutral molecule of ferrous oxide ($FeO$), we know that the charge on Oxygen is $ - 2$ . Thus,
$\left( {1 \times x} \right) + \left( {1 \times - 2} \right) = 0$
we have multiplied $x$ and $ - 2$ with $1$ as iron and oxygen have only one atom each in $FeO$
On solving it further, we get,
$
  x - 2 = 0 \\
  x = + 2 \\
 $

Thus, we can infer that the oxidation number of iron in ferrous oxide ($FeO$) is $ + 2$.

Note :
 The terms oxidation state and oxidation number are also used interchangeably since they have the same meaning for atoms in a molecule. It doesn't matter if the word oxidation state or oxidation number is used much of the time. The value of oxidation of an atom in a molecule is referred to as the oxidation state.