There is no set number. The weights vary as does ankle resistance. I would recommend starting from the lighter end to get feel of how comfortable they are then start adding the weight. Remember it is easy to tear a tendon - so go easy and gauge your resistance.
It depends what weight of ankle weights you are using. If you are using light weights, you should aim for more repetition (15-20 reps). If you are using a heavier weight, you should aim for less repetition (5 reps).
Assuming you do not have any medical issues, you should start with a set of 15 leg lifts or until the muscle feels tired. You can increase the number of sets or the amount of weight as you get stronger.