Speaking of IAL I should be getting my shipment in today or tomorrow.. got a 100 grade A large (8" - 12").
Depending upon size it will depend upon how many you place in the tank - and how dark you want the tank.
I would just use one to start with and see how you like it.. if you wish it to be darker you can create your own tannin and add it in the tank along with the one leaf. Just crush up one leaf and place it in a gallon water jug with water and let it sit for a day.. then use that water during a water change.
You can also purchase them on Aquabid.. this is where I get mine - tantora