The given equation is follow,
2 Cl₂ + 2 KI ⟶ __ KCl₂ + I₂
There are two Iodine atoms at left hand side of the equation and two at right hand side of the equation so Iodine is balanced.
There are four chlorine atoms at left hand side and two at right hand side. In order to balance chlorine atoms we will multiply KCl₂ on right hand side by 2. So,
2 Cl₂ + 2 KI ⟶ 2 KCl₂ + I₂
Now, the potassium atoms are same on both sides, i.e. two.
Result:
The missing coefficient in given equation is 2.