how many books in a series?

Now that is a question, is not it. Sometimes, I get tired of a series after the first one. An example is the Eragon's series. On the other hand, I like the Wheel of Time and 12 books up to this point is just fine. I will read the rest of it, even if Robert Jordan is dead. Anyway, I think three books is about right, like Lord of the rings.  Is one book in SFF ever successful? Probably not. I will target three to six books.