The charge of an electron is approximately -1.6 x 10^-19 Coulombs. To find the number of extra electrons, we can divide the given charge by the charge of a single electron:

Number of extra electrons = (-21mC) / (-1.6 x 10^-19 C)

= -21 x 10^-3 C / -1.6 x 10^-19 C

= 13.125 x 10^16 electrons

Therefore, an object with a charge of -21mC has approximately 13.125 x 10^16 extra electrons.