Finding the valence electrons is much more complicated for transition metals. Take a look at this resource: https://blog.udemy.com/how-to-find-valence-electrons/
"For the transition metals (groups 3-12), figuring out the valence electrons is more complicated. Their atomic structure is such that their d subshell is incomplete. This means that a shell that sits lower than the outer shell will be where the valence electrons react. It gets complicated.
But not all “valence shells” will actually have valence electrons! Sometimes these shells will be filled, or closed, which means that they have no electrons available to interact with. These atoms will not react or form bonds and so are called inert (you can call someone who sits on the couch and watches TV all day inert as well, but this has nothing to do with chemistry). All the noble gases in group 18 fall are inert.
A way to find valence electrons without the periodic table is using the atomic number and drawing a diagram. The atomic number is how many protons and electrons the atom has. That means an atomic number of 8 (oxygen), has 8 protons and 8 electrons."
It should clear things up.