Answer
A $Si$ atom has 4 valence electrons.
Work Step by Step
In most cases, the valence electrons are also the electrons in the outermost occupied shell.
Here, in the $Si$ atom, the outermost occupied shell is shell $n=3$.
There are 2 electrons in subshell $3s$ and 2 electrons in subshell $3p$. That means, overall, there are 4 electrons in shell $n=3$.
Therefore, there are 4 valence electrons.
