The end is everything.
Mickey Spillaine, author of the Mike Hammer books once said,
'They read it to get to the end. If it's a letdown, they won't buy anymore. The first page sells that book. The last page sells your next book.'
If you have planned your series arc to last six episodes or eight episodes or whatever, and you suddenly find that you are selling well, there is a temptation to say to yourself, 'You know what? I could stretch this out a little. Make more money.'
Don't do it. Stick with your original plan and end the series. If you really feel like there is more mileage in the characters and story, write a second season and explore your fictional world deeper there. But make sure that the series you are writng now has an ending.
Readers want closure. They won't follow a series that never ends just because you want to string it out and make more money.
You must look at each 'season' as a self-contained unit. Your readers want closure. No matter how much they enjoy your story, they want to know what happens in the end.
Have you ever told a friend about a movie you saw? Describe the important plot points to them and they will most likely ask this question:
'What happens in the end?'
That should be the question in your reader's mind as they read your story. How will this end? What will happen?
Don't cheat them out of the answer. You are leading your readers along a path through a world of your creation but always remember that you are leading them somewhere. Every journey ends with an arrival. Let your readers arrive at their destination...the end of the story. Give them closure and leave them wanting more.
Blow them away with a killer ending and you will have them clamoring for your next book.
Your readers have come this far with you...give them the finale they deserve.