There are 3xx7+5=26 valence electrons to distribute, i.e. 13 " electron pairs".
Explanation:
Around EACH bound Cl atom there are 3 lone pairs; there are 3xxP-Cl bonds; the thirteenth lone pair resides on phosphorus: :P(-Cl)_3. Since there are 4 electron pairs around phosphorus, the geometry is based upon a tetrahedron, but since one of these electron pairs is a stereochemically active non-bonding pair, the geometry around phosphorus is described as trigonal pyramidal.