You can weigh any number of balls per try, it does not have to be all 8.
Start with an equal number of balls on each side of the scale.
Start 3 with balls on each side of the scale.
You place balls 1, 2 and 3 on one side of the scale. On the other side, you place balls 4, 5 and 6. If the scale shows them to be of even weight, discard all 6 balls. Weigh balls 7 and 8 and you'll know which is lighter. If balls 1, 2 and 3 and 4, 5 and 6 are not equal weight, take the 3 balls that weigh less. Place 1 of the 3 balls on one side of the scale,and 1 on the other side. If one is not lighter than the other, the remaining ball is the lightest!