I am not familiar with demographics for ortho offices, but from what I have seen, a ratio of 1 ortho per 8,000-10,000 people is good. However, a small town of 10,000 will probably have 5 or so dentists, but may not have the income required for an orthodontist, so pay attention to that too. A median income of $60k+ is preferable.
Also keep in mind that rural dentists have to be able to do it all, since their patients may not be able drive into a larger town to see a specialist. They may already be doing ortho in-house.
I would strongly advise against an ortho start-up. It can be done, and very successful, but you'll be eating beans and ramen for a while before your patient base is large enough to have case starts on a regular basis. It is much safer to buy a retiring doc's practice and have those referral connections already set up. Even if the selling doc wants to finish his cases, you'll have a steady stream of referred patients and enough case starts to have a predictable income. Even if you want to move locations, change the office up, hire/fire staff, you'll be able to sleep soundly if you have a steady income.