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Valence electrons are those after the preceding noble gas. Therefore, if you write the electron configuration for the element, whichever electrons come after the preceding noble gas are all valence electrons. For example, for manganese, the electron configuration is [Ar]3d54s2. Therefore, manganese has 7 valence electrons.
You only count the s and p block electrons. You do not count the d block electrons because the d block would not be part of the outermost shell. For instance, with gallium, [3d104s24p1], the s and p are in the same larger shell than d (4 compared to 3). For transition metals, groups 3-11, they are allowed to use the d orbital as part of their valence electron, so manganese can have 2 or 7 valence electrons.
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