The total number of electrons that take part in forming bonds in a ${\mathrm{C}}_2{\mathrm{H}}_4$ molecule is_________

For ${\mathrm{C}}_2{\mathrm{H}}_4$ you have a total of 12 total valence electrons. 

According to Lewis structure of ${\mathrm{C}}_2{\mathrm{H}}_4$, It has a double bond(1sigma,1pi) between two carbons and each hydrogen atom involved in a sigma bond with C.

Hence, total electrons present is 12