You should have a realistic expectation that whilst 80%+ clearance is common; 100% disappearance is rare. You should also be aware that the procedure treats only leg veins that are currently visible.; it does nothing to permanently alter the venous system or prevent new veins from surfacing in the future.
Sclerotherapy is a treatment for the symptoms rather than the cure for the cause, other veins may develop. Most people need a top up session every 1-2 years. If you have underlying varicose veins, you are recommended to see a Vascular Specialist to treat the larger vessels before treating the smaller superficial veins.