An atom becomes negatively charged when it gains electrons. The number of electrons an atom needs to gain to become negatively charged depends on the number of protons in its nucleus, which determines its atomic number and the number of electrons it normally has in its neutral state.
In general, if an atom gains n electrons, it will have a negative charge of -n. For example, if an oxygen atom (atomic number 8) gains two electrons, it will have a negative charge of -2.
Therefore, the least number of electrons an atom must have in order to have a negative charge would be one more than the number of protons in its nucleus, since adding one electron will give it a charge of -1. For example, if the atom has 6 protons, it would need 7 electrons to have a negative charge of -1.
This corresponds to the element carbon, which has atomic number 6 and normally has 6 electrons in its neutral state. Adding one electron to a carbon atom would give it a negative charge of -1.