To determine the oxidation number of chlorine in MgCl2, we can use the following steps:

1. The oxidation number of magnesium (Mg) is +2.

2. The overall charge of the compound MgCl2 is 0.

3. Let the oxidation number of chlorine be x.

4. We can set up the following equation: (+2) + 2x = 0.

5. Solving for x, we get x = -1.

Therefore, the oxidation number of chlorine in MgCl2 is -1.