How do I decide how many books will be in a series?

My assistant and I usually sit down and plan out every new series and it always varies how many main characters we’re going to deal with and how many books that means. Most of the time I stick to that number, but there have been plenty of times when I’ve added on to a series that I never planned on going beyond three books! When I wrote the first three Montgomery Brothers books, it was only supposed to be a trilogy. But it was so successful, it ended up going to ten books, and then I just released an eleventh! Magnolia Sound was planned to have six books, and we ended up with eleven there too. But there was a time when I thought it would go on for at least fifteen. Adding on to an existing series is really based on what the readers want and how well the books are selling, but I’m learning to start with a conservative number and then go from there.