There are two ways we can determine the Valence Electrons of an element, one of these is with the use of electron configuration.   If you are using electron configuration, you need to determine the highest energy level and count the number of valence electrons found in the highest energy level.  But these is only true for those Representative elements or the group A elements.

Let us have an example

1.  H, atomic number is 1

     Electron Configuration:   1s¹

     Since hydrogen has only one electron, therefore the valence electron of hydrogen is only 1.

2.  He, atomic number is 2

     Electronic Configuration:  1s²
     Valence Electron :  2

    Since helium has only one sublevel and orbital, valence electrons is 2.

3.  F, atomic number = 9
     Electron Configuration:   1s² 2s² 2p⁵
     Valence electrons :   7

     The valence electrons of Flourine is 7 , the highest energy level is 2 having two sublevels the s and p.  Two electrons in 2s sublevel and 5 electrons in 2p sublevel, which equals 7 valence electrons.  We need to add since both are in the same energy level.

4.  Ca, atomic number = 20
     Electronic Configuration:  1s² 2s² 2p⁶ 3s² 3p⁶ 4s²
     Valence electrons:  2

5.  Al, atomic number = 13
     Electronic Configuration:  1s² 2s² 2p⁶ 3s² 3p¹
      Valence electrons:    3

Relationship Between the Valence Electrons and the Group Number

Representative elements (Elements from group IA to group VIIIA now group 0) valence electrons are related to their group number.  Based from the examples above, Hydrogen being in group IA has 1 valence electron, Flourine in group VIIA has 7 valance electrons, Calcium in group IIA has 2 valence electrons and aluminum group IIIA has 3 electrons.  Except helium which is an exception. In other words, valence electron is just equal to the group number of elements in Representative group of elements but not related to transition elements.