welcome to GoY.
one tuber per slot as they produce large plants. I 'd use a good quailty multi purpose compost and add the slow release plant food granules. To help with the watering I'd also add water retaining crystals as the pouches are likely to dry out quickly.

Also mix your compost with vermiculite this will help with retaining moisture as I found out with mine last year these bags get very warm and need a lot of water.