Answer:
six electrons need to be gained
Step-by-step explanation:
Calcium has two valence electrons. To have a stable amount of valence electrons, or to attain noble gas electron configuration, an element needs to have eight valence electrons. Now, it's just a simple math problem. How many more valence electrons does calcium need to get to eight? It needs six, since eight minus two is six.
Hopefully that's helpful! :)