How many electrons would be expected in the outermost electron shell of an atom with atomic number 12?

First, what does the atomic number 12 tell us about the atom?  Well, it tells us the number of protons(+) that atom has. If the atom is not bonded to any other atoms the number of electrons(-) is usually equal to the number of protons. Therefore, we can assume this atom has 12 total electrons. 

Next, it is important to understand electron shells.
The first electron shell around an atom always has 2 valence electrons with the exception of Hydrogen because it only has one electron to begin with. It doesn't have enough electrons to fill it up with 2e-. 
Every electron shell after the first shell fills up with electrons until it runs out of electrons or until it reaches 8 electrons. Once a shell fills up with 8 electrons a new shell is started. This pattern continues until all the atom's electrons are used up.

I drew a picture of this atom and its electron shells so that you can see what I mean. The answer is: The outermost shell will have 2 electrons on it. The electrons on the outermost shell are called valence electrons. 

If you understand the material, often times you can use a shortcut for this problem. (atomic #)12 - (shell 1)2- (Shell 2)8= 2e-    You keep subtracting 8 until your answer is less than or equal to 8.

Hope that made sense! Comment with any questions if things are still unclear or if you think I made a mistake. :)