A ion of the element oxygen (o) has overall charge -2. Therefore, the number of elections in this electrons in this oxygen ion is

18 electrons

Explanation:

Each electron carry a -1 charge.

Since Oxygen atom originally has 16 electrons (same no. as no. of protons, where net charge as 0), it must add 2 electrons to bring the net charge to -2. Hence, the number of electrons in this ion is 16 + 2 = 18.