How many books should be in a series?

As an author of both children's and adult's fiction this is a quandary. Though each book in the Friday Night's Dream series is able to be read individually the series arc has a definite ending. This is planned and could come in book four or five. If new plots come to mind this number could be extended. The third book, Dark Clover, spans less than a year with this in mind but I don't really want to broaden the stories beyond the enchanted city of Wyckerton so will stop when that becomes a challenge.
As I wander around my magical city I find stories that suit other characters. When I've finished Alfie's story there are many others lurking in parts of Wyckerton.
Aunt Edith's for example. She is an old Grandma but it's mentioned that when young she was a whizz at broom riding and a champion Manhunt player.
Going back even further, the story of the rivalry between Wyckerton’s famous two brothers, Martyn and Mervyn, calls to be written.
Tell me your thoughts; otherwise I can just continue to write as I wish!