For the surgical subspecialties orthopedics, ENT, plastics, and urology, it is a good idea to apply to as many programs as possible - say 50-100. (assuming you aren't AOA). The same is true for derm and radiation - apply to as many as possible. It is expensive but you won't regret it on match day. You can still go unmatched even with 10 interviews in these fields, so the more chances of getting interviews, the better.
For intermediate-competitive fields like emergency, gen surg, ophtho, pick an area of the country and apply to the majority of those programs. For these fields I've heard it is ideal to have 10-15 interviews, depending on the strength of your application.
Not as competitive fields include IM, peds, med-peds, anesthesia, path, FP, psych. Still if you apply to top-tier programs these can be very competitive so be sure to apply to several middle-tier programs. No matter what, I would still avoid interviewing at less than five programs. And rank all programs you interview at.
In general it is safer and prudent to apply to slightly more programs than you might expect to interview at. It is always easier to cancel an interview or stick a program at the bottom of your ROL than to go unmatched!
Another thing to consider is having one "practice interview" at a neighboring institution you don't plan on ranking highly. Another thing is to always schedule your interviews ASAP - at times spots will literally fill up in a matter of hours after the email is sent. Check your email regularly.
