I am not totally sure... A 9 liter bag would probably be best, if not you can always save it for another tank.
Just a heads up, anubias and java moss are not going to benefit from Aquasoil that much. It is a waste to spend that much money on substrate for those plants. Fluval makes stratum as well and it is much cheaper, you would need like the 8.8 pound bag of it, maybe a tiny bit more.
Identical look, way cheaper, not as good but still great.
Keep researching aquasoil, you will find more to it. Like did you know initial setup the first week you need to be doing daily 50%s to keep ammonia down, then 50% every day the next week? It is a lot of work, and I it will take super long to master keeping those awesome ADA aquariums.
in my 48g I use aquatic potting media, My anubias grow extremely well and the pond Lilly I added the other day is also growing really fast. it needs a very good rinsing before use, it will be very dusty.
there are other plant gravels or even potting soil you can use, the potting media was cheap and seems to do a good job :)
You can look into eco complete, but I can't guarantee that anything will grow. It depends on you really. I would start out with much easier plants. Water wisteria, ludwigia, rotala, dwarf sag, Crypts, Anubias, and java moss are all easy plants. Start out easy, you will feel much more confident seeing a beautiful tank, instead of one that is barely surviving.