In the second column of the periodic table we find the following elements: Be, Mg, Ca, Sr, Ba Ra. To identify which element you are referring to we must look up its atomic number.
The atomic number corresponds to the number of protons, so we will look for an element whose atomic number is equal to 20.
It corresponds to the element calcium, which has an oxidation state equal to +2, that is to say, that its ion must lose 2 electrons to have a positive charge equal to 2.
As a neutral atom has an equal number of protons and neutrons, the neutral atom of Ca will have 20 electrons and the Ca+2 ion will have two fewer electrons, that is to say, 18 electrons.
The ion has 18 electrons