Cruise ship brochures are full of seductive, enticing photos of destinations, life on board the ship, and inviting shore excursions. And then, there are the itineraries themselves.

Once you get past the photos (can we ignore the parade of happy couples toasting wine glasses?) and the cost of the cruise, you normally focus on the itinerary itself and the ports of call.

But before you pick out your cabin category and all the other ancillary options, you might want to reverse the process. You need to figure out one important — and defining — item on the itinerary you prefer, and in each port where you're stopping, how many other ships will be there.