Can I have too many or too few characters?
You can have as many characters as you like in fiction. Sometimes you need at least two but it´s not essential. I can´t think of many good examples of fiction in which there is only one. You tend to need two because you need a conflict. What you do need to do is make sure that each character that you have introduced is fully developed, even the minor ones. Probably Dickens is a very good example of a writer who is very democratic to all the characters that he creates, and he creates many.