How do you figure out how many Valence electrons is in an atom?
The number of valence electrons that an atom has is equal to the group number of that atom.
The group number of a certain atom is equal to that atom's number of valence electrons only when you are referring to main group elements. The main group elements are groups 1,2, 13-18, with the exclusion of the transition and inner transition metals. I should also add that helium only has two valence electrons even though it is in group 18. This is because helium's electrons only occupy an S orbital, which can hold a maximum of two electrons.
For example, let's say that you have a Ca atom. Since calcium is located in the second row of the periodic table, it will have two valence electrons in its outermost shell.
Here is a more detailed explanation with examples!