A valence electron, is an electron that can form a chemical bond with another atom/molecule, just like you know it from the shell model.
It is not any different, when you use orbitals, but there is of course some probability of finding the electron outside its orbital (in another orbital), which is described by quantum mechanics. The orbitals also overlap each other. But the way you count the amount of valence electrons, is still the same, because valence electrons will have a higher energy, and thus they will have the highest probability of being found in the outer most orbitals. It is also these orbitals, that are energetically unfavorable, and thus the atom will either need more energy or want to let some go, by accepting or donating an electron.
Of course once you start to look at "real-life" problems such as bulk compounds it becomes a bit more difficult, since the way two atoms form bonds can result in even more valence electrons, and thus give rise to conducting properties.
Also you have some metals, that can have valence electrons in more than one shell in the shell model, but I would recommend reading more into that stuff.
As a last note, I could mention the Pauli principle, which says that you cannot find two electrons at the same place. This also leads to the fact, that the probability of finding the electrons near each other is low. So the average locations of the electrons will be kind of like that in the shell model. This is also, why you can talk of a filled orbital, and you can use this to even explain the shape of the orbital depending on the amount of electrons.
Of course the mathemathics is not as "simple" as that, and it is after all, the math that proves it..