The carbon with a negative formal charge will have 1 lone pair of electrons.
Work Step by Step
Normally, a negative carbon will have 2 free electrons, and it will make 3 bonds (Since it has 5 (4+1) electrons in the valence shell). The octet rule states that a carbon can't have more than 4 bonds. For this reason, a carbon with a negative formal charge can't have 5 bonds and no free electrons.