In chalcone You have C, H, and O atoms. Carbon has 4 valence electrons, hydrogen has 1 and oxygen has 6. Simply add electrons from each atom in the molecule.
For example nitrogen has 5 valence electons because it belongs to V group.
For organic molecules you need to calculate the oxidation number for carbon atoms depending on electronegativity of the atoms attached to carbon atom. With high electronegative atoms carbon take a positive charge and with lower electronegative such as Hydrogen carbon take a negative charge as an example the oxidation number of carbon atom in a simple molecule CH3OH is -3+1=-2 and +4 in CO2.