I never have a solid number of chapters. The number of chapters is completely unimportant: it’s just a way of subdividing a story, and the story’s the thing. You use as many as you need to tell the story.

I usually like to have a complete outline that will take me through to the end. I have to admit though, my outline for The Grave Thereafter takes me up to a little after the climax. (My main issue with this story is that I’ve got a lot going on — maybe too much — after what I consider the climax of the story). But right now, it’s exactly as you say: I’ve got enough to get started.

The one rule of the outline is that it’s never, never set in stone. If, while you’re writing, you come up with a twist, or an extra chapter you want to work in, by all means, do it. If I was more organized (/anal-retentive) I’d work that extra chapter into the outline with a paragraph of its own. I never do. I just write the thing.

If you find the outline becoming a crutch, start writing.

The writing is the all that matters. You can spend some time finishing your outline after you hit your word/time goal for the day.