If we look at the periodic table we will see that P belongs to group 15 or 5A. That means that it has 5 valence electrons.
Br belongs to 17 or 7A. That means that it has 7 valence electrons.
Our molecule is PBr₃. One molecule of it has 1 atom of P and 3 atoms of Br. Each atom of Br has 7 valence electrons and the atom of P has 5 valence electrons.
Electron count = 1 * 5 + 3 * 7
Electron count = 26 valence electrons.