On my last post, David commented on the Wikipedia's attempt to classify airports into "private" and "commercial." As David pointed out, that is nonsense: they aren't even opposites. A 1500' private grass strip can be dedicated to a commercial agricultural spraying operation. Any runway that is open to the public for landings and takeoffs without prior permission probably hosts some sort of commercial air operation: I can't think of one that doesn't, even the unpaved ones. And there is usually some kind of commercial activity going on anywhere. I know of a private grass strip whose owner rents out hangar space, and pretty much every public aerodrome has paying commercial tenants.
So you could try to classify airports as private versus public, but where would you put those that are open to the public but prior permission is required to land there? Or the ones that are open to the public as long as prior notice is given of the intent to land? Or the ones that are open to the public, but it's public knowledge that they get ticked off if you land there too much? It's a continuum of access, not a dividing line.
But there are ways airports are classified. In Canada, anywhere that is used or set aside for landings and take offs is an aerodrome. Yes, anywhere, be it land, water, or the frozen surface thereof. The documentation itself admits that by that definition most of Canada is an aerodrome. If that aerodrome is listed in the Canada Flight Supplement, then it's considered a registered aerodrome. An aerodrome is listed in the CFS if its owner submits it, it meets minimum safety standards, and it's a land aerodrome. (Water aerodromes have their own book. That's another way to classify them: land vs. water). And if the aerodrome meets a more stringent set of safety standards, it becomes a certified aerodrome. And that's the Canadian definition of an airport: a certified aerodrome. You'll notice that I sometimes use the words airport and aerodrome interchangeably. I can't be bothered to look up whether an aerodrome is certified or not before talking about it.
Sometimes when people ask me if I want to fly commercially and I tell them I work for a commercial operation now, they reply, "you know what I mean!" And I do. A commercial airplane is a big one that you pay to get on with a lot of other people and fly away on vacation. When the public says "commercial" airport they mean it the same way as they mean commercial airplane. A commercial airport is where you board a commercial airplane, in order to start your vacation.
So how many people is "a lot"? The Canadian Aviation regulations do include some numbers and airplane weights and configurations that form the boundaries between one kind of operation and another. If you're a Canadian commercial pilot, you know what they are, so just sit smugly. Otherwise, hit the comments and tell me how many seats you think an airplane has to have before it's big enough not to ask the pilot if she'd like to fly commercially someday. I'll ask my mom, too.