Hallo Melody,
$$\small{\begin{array}{rcl}\mathbf{ \varphi(12)} &\mathbf{=}& \mathbf{4}$\\
\end{array}}$$,
because
$$\small{
\begin{array}{r|rcl|c}
\hline
\text{number} &\text{greatest common devisor} &&& \text{relative prime, if 1} \\
\hline
1 & \gcd{(12,1)} &=& \textcolor[rgb]{1,0,0}{1} & \text{yes}\\
2 & \gcd{(12,2)} &=& 2\\
3 & \gcd{(12,3)} &=& 3\\
4 & \gcd{(12,4)} &=& 4\\
5 & \gcd{(12,5)} &=& \textcolor[rgb]{1,0,0}{1} & \text{yes}\\
6 & \gcd{(12,6)} &=& 6\\
7 & \gcd{(12,7)} &=& \textcolor[rgb]{1,0,0}{1} & \text{yes}\\
8 & \gcd{(12,8)} &=& 4\\
9 & \gcd{(12,9)} &=& 3\\
10 & \gcd{(12,10)} &=& 2\\
11 & \gcd{(12,11)} &=& \textcolor[rgb]{1,0,0}{1} & \text{yes}\\
12 & \gcd{(12,12)} &=& 12\\
\hline
&&& \text{sum} &\textcolor[rgb]{1,0,0}{4}\\
\hline
\end{array}
}$$
