It will use all task slots and it will do the Profession that is the bottom of the list as many times as you have assets for. It will always fill all assets slots that you have assets for as well, just make sure you sort them in game in the order you want them used.
eg. I work on Leadership, Leatherworking, and Tailoring on one of my characters. When all Professions are the same level this will always work on Tailoring first, then Leatherworking, then Leadership. Once tailoring levels up then it will be Leatherworking->Leadership->Tailoring