The number of valenc electrons found in a representative element is determined by which group it is in in the Periodic Table.

Hydrogen is in Group IA so it has 1 valence electron

Carbon is in Group IVA so it has 4 valence electrons

Nitrogen is in group VA so it has 5 valence electrons

Oxygen is in Group VIA so it has 6 valence electrons

Chlorine is in Group VIIA so it has 7 valence electrons.

So the molecule NH3 has 8 valence elctrons total (five from nitrogen and one each from three hydrogens)

HCl has 8 valence electrons

CO2 has 16 valence electrons

CH3COOH has 24 valence electrons (8 from carbons, 12 from oxygens, 4 from hydrogen)