Excellent! As you can see in the figure, there are a total of $$4$$ electrons in the Beryllium atom. So, its atomic number is $$4$$.
$$\text{Mass Number}$$ = $$\text{No. of protons}$$ $$+ \, \text{No. of neutrons}$$
For example, as there are $$4$$ protons and $$5$$ neutrons in Beryllium, Beryllium's mass number is $$4+5=9$$. It is called the Mass Number because the mass of an atom is determined by the number of protons and neutrons in it. The electrons have a much smaller mass, so, their contribution is negligible.