Entering Canada on a visitor visa: is there just one airport I can go to?

There is no limitation that I can see on where you enter Canada on a visitor visa, any international airport should do. It is possible slightly different conventions apply if you were thinking of crossing from the US or entering by sea but I haven't found anything official to suggest that is the case.

You will have to give them a "purpose of travel" in the application process which might include your sightseeing itinerary (e.g. you are travelling to Calgary, X, Y, and Z locations for a holiday). They don't care how you organise it specifically as long as it seems reasonable.

So if you arrive in Vancouver with plans to holiday in Calgary, they may ask what your onward travel plans are (e.g. are you flying or taking a bus or if you're really feeling ambitious, driving yourself). They'll probably ask where you're staying and maybe what local sights you're interested in. They may want to see your hotel reservations and return flight reservations.

But they won't limit exactly how you enter and leave Canada, since it's not uncommon for people to choose a particular route for convenience or cost. The only issue would be if you were heading onwards to another country from Canada, not returning directly home - then they may want to see some proof you'll be legally able to enter there. For example, if after Canada you were heading to the US, or transiting via the US, you should have your US visa (if needed) already sorted.

If you have a Visitor’s Visa to Canada stamped inside of your valid passport, you can enter Canada at the Calgary International Airport, the Edmonton International Airport or any of the several other Canadian international airports that you decide to book a flight for. Although Canada’s many ports of entry are staffed by members of the Canada Border Services Agency (CBSA), who will check your travel documents and make the decision regarding authorizing entry into Canada, some Canadian airports may be open limited hours. Some of the most popular ports of entry for Canada include the international airports for Toronto, Montreal, Vancouver, Calgary, Edmonton, Ottawa, Winnipeg, Quebec City, Halifax, and Victoria.