A journal can carry at most two disciplines. There is no field for a third: the row only has room for a first choice and, once that is set, a second one next to it.
The second field only shows up after the first is set
Before you pick a discipline, you see one dropdown. As soon as you choose your first discipline, a comma and a second discipline field appear next to it. That field is not hidden or broken before then, it simply has nothing to attach to until a first discipline exists. If you are looking for a way to add a discipline and only see one box, pick that one first and the second appears on its own.
The second list will not offer you your first choice again
Once your first discipline is set, the second dropdown's list leaves that discipline out entirely. This is deliberate: a journal cannot carry the same discipline twice, so the option is removed rather than shown and then rejected. If the discipline you are looking for in the second field is the same one you already chose first, you will not find it there. Search a different discipline, or change your first choice if that is the one you actually want in the second slot.
Going back and duplicating it anyway is refused, not saved
The one place a duplicate can still be attempted is by returning to the first field and reselecting whatever you already have set as your second discipline. That is refused: you get a toast that says "Please select a different discipline," and the first field snaps back to what it held before you touched it. Nothing saves in a half-set state, and no request goes to the server for a duplicate pair.
Clearing your only discipline blocks publishing
Both discipline fields carry a clear button. If you clear the only discipline you have set, the journal drops to zero disciplines. A journal needs at least one discipline, a story type, a title, a tagline, a cover image and body content before the publish button will work, so an empty discipline field is one of the reasons publishing gets refused until you set a discipline again.
The dropdown right before your disciplines, labelled "under," is the story type field. That one is covered separately in How the journal story type works.