David,
You haven't answered my question.
For YOU, how many holes (if not 1) does it take to make up a course?
Oh. A course, or a great course? There's a 12-hole course near me that I play from time to time, which is fun enough that I can play a round and leave, but nobody would ever call it great. But it is a course.
We built Stoney Hill a hole at a time. At various times it was a 7-hole course, an 11-hole course, a 13-holer, a 17-holer.....but when we got to 18, it seemed like a full course. Those shorter versions were full of good holes, but they didn't quite seem complete.
There's nothing magic in the number 18, other than tradition---and the fact that so many other courses we play are 18 that it seems like a full course. 9-holers are rare around here and I played at least 10 years before I ever saw one.
That's one limitation of a 9-holer. The other, in regard to a great course, is a question of how much variety can it have? It's easier to get a wide mix of lengths, of elevation changes, of straight shots, RHBH hyzers, RHBH anhyzers, LHBH hyzers, LHBH anhyzers, ace-runs, par-3s, par-4s, par-5s, tight wooded shots, more open carved-through-the-woods shots, open-with-obstacle shots, open shots, big downhills, low ceilings, easy flat open greens and obstacle-filled straddle-putt greens and fast hillside greens, and all the other features you might incorporated, if you have 18 holes to do it in. Or, at least, as many as possible. To come up with 9 holes and present the same variety of challenges, and require the same range of shots, is a real test.
The best example of the potential I can come up with is the back-9 of Ashe County. I rate Ashe County a 5. If the front 9 were taken away and the back-9 kept as a 9-holer, I'd give it a 4......maybe a 4.5. That's probably my ceiling.