It all depends on what kind of situation you think you'll learn well in... you've answered your own question somewhat. I wouldn't be "leery" of any program unless they repeatedly fail to match or you've heard bad things from multiple residents/externs who worked there.
Programs that only take 1 mean you'll be on call a lot (esp when one of the others is on an external rotation). It also means you want to be a pretty good self directed learner and researcher since there's not much competition to push you or many other residents to help you on projects and lit reviews. I would guess that going to a program with only one resident also means your job is pretty secure since they'd be even more understaffed if they fired a resident. You'd also get to know the attendings very well in research, but some programs that only take 1 or 2 per year sometimes don't have a whole lot of attendings and you had better make sure they are good ones you want to emulate. You won't really benefit with a limited number of attendings if you are the kind of student who likes to learn a couple different tricks from each pod and move on.
Programs that take a lot of residents can do that because they have a lot of surgery volume and attendings (or at least they should) and COTH therefore accredited them for more spots. The downside is that some of those programs are then "top heavy" where you won't get much or any C level surgery or quality time with attendings since the numerous senior residents and fellows may take priority on cases, take credit for some of your research work, etc (of course you will get to that same status someday, though). A lot of programs that take 3+ per year split their residents up between various nearby major hospitals and only have first year residents take call, so it might not be as overcrowded or as easy of a call schedule as you think. The pressure and competition of having many other pod residents around might make some people rise to the challenge and do their best work, but it might be discouraging to others. Another potential downside is that a big program that has a lot of residents might have no problem cutting a couple stragglers loose since there are plenty of other stronger residents to pick up the slack and workload. No, nobody sets out to be the worst resident in their program or way below their peers, but sometimes people will end up as just that, esp in some elite ones.
Word of mouth is always good to narrow the 200+ residency location choices to the 5 or 10 you'll visit, but I think the externship is ultimately the best way to figure some of these things out and assess first hand the program case load, teamwork, strengths/weaknesses, etc. It might seem pretentious, but I'll be analyzing them as much as they are me. It's 3 years of your life and will greatly influence how happy or miserable you might end up in your pod career....
