Well, even though these bonds are ionic, the valency still deals with how many pairs of electrons the atom can share. All of the alkali metals are considered to have "one" valence electron since they have their S shells half filled. Therefore, they are willing to bond with something that will give them a full shell. When they bond with the chlorine atom, they are able to donate their electron to chlorine thus given both atoms a noble gas configuration. With Calcium, there are two S-orbital electrons which can be "donated" so now two chlorine atoms can take an electron and wind up giving all three atoms the noble gas electron configuraiton. This is why Calcium, Magnesium, Strontium and Barium form halides which have two halogen atoms in there.