You don't need a minimum number of people to create a social group.

You do need a minimum of 25 social group members in order for that social group to create it's own forum. (I think the OP was referring to a social group forum, but was using the word "thread" instead to describe his need or desire just to have a private place to post his thoughts.)

If you want someplace truly private that no one else can access, simply send yourself PMs. That way they will be there only for you and your use.