There are two basic uses of magic in literature. One is as a catalyst for a cautionary tale on the dangers of power. Examples of this include The Lord of The Rings and A Wizard of Earthsea. The former is a treatise on the nature of temptation, and the latter on the nature of pride.
The second use is to indulge the reader's fantasies of specialness. Power qua power is not really the point here. It is really all about claiming special status, a reason why the rules that apply to ordinary people do not apply to you, and having magical powers, particularly world saving magical powers, is an obvious source of specialness. Harry Potter is the preeminent example of this.
The latter is far easier to write, especially if you aim is to do a series. BTVS started out as more of a cautionary tale and became increasingly about wish fulfillment after season two.
The point of making this distinction is that the answer to your question depends on it. There is actually remarkably little use of magic in LOTR and none of it is explained. There is a little more in Wizard of Earthsea, and it seems to be vaguely of the knowing the secret names of things variety, but again it is not explained. The operations of magic are not the point in these stories, it is the moral challenge that they present to the characters that matters.
In the fantasy of specialness kind of story, however, the operation of magic seems to matter a great deal. That is because the almost inevitable progression of such stories involves a process of "learning to master your powers". (As soon as someone says "your powers" you know you are reading a wish-fulfillment fantasy.)
The culmination of the story in these works often depends on the correct manipulation of magic (as opposed to the mastery of your moral weakness in the cautionary tale) so again, the operation of magic matters.
These kinds of magical wish fulfillment stories are almost always written as coming of age stories, because they are essentially pandering to a childish wish to be special. How much the details of the magic matters is going to largely depend on how central it is to that coming of age.
In either type of story, the magic is a McGuffin. It does not matter in itself. It matters because it fulfills a function in the story, either as a source of temptation, pride, or danger in the cautionary tale, or as a sign of specialness in the fantasy of specialness. You need as much or as little magic, and that magic needs to be as vague or as specific as it needs to be to fulfil its role in your particular story.