Calcium atoms will lose two electrons in order to achieve the noble gas configuration of argon. A neutral calcium atom has 20 electrons, while a calcium atom that has lost two electrons will have 18 electrons, and a neutral argon atom also has 18 electrons.
Calcium has two valence electrons in its last energy level.
It is easier to lose two electrons to adquire stability in order to achieve the noble gas configuration.
Thus, calcium will lose two electrons to get a full outer shell.