The bolt of lightning transfers a charge equal to 25 C to th Earth. An electric charge is expressed in Coulombs (C). It is negatively charged particles or electrons that move when a current travels though this is equivalent to a positive charge moving in the opposite direction.
To determine the number of electrons that are transfered to the Earth use the electric charge of an electron. The magnitude of the charge is equal to 1.602*10^-19 C. (As electrons are negatively charged the charge on each electron is actually -1.602*10^-19 C) The number of electrons that carry a charge equal to 25 C is 25/(1.602*10^-19) = 1.56*10^20