Heyy Rohan....

Totally 12 electrons are donated to Pt. Each ligand donate 2 electrons 
First count the number of ligands. There are 2 en (ethylene diamine) which is a bidentate ligand. So its contribution of electron is 4 x 2 = 8 electrons. 2 Cl contribute 2 x 2 = 4 electrons. So totally 12 electrons to the central metal atom.   

Hope it helps !!