They continue spawning indefinitely and there is an unlimited number of them that can play the game, but in order to encounter an "INFINITE" number of dragons you would have to continue playing Skyrim for an "INFINITE" period of time.
You can play the game for 500 hours, 1000 hours, 10,000 hours, you are still not going to run into an "Infinite" number of dragons.
Who knows how many you personally will encounter during your playthrough of 100, 200, or 300 hours. perhaps you will encounter 10, or 20 or 30 dragons during that time. The point about having an unlimited number of dragons just means you can keep playing the game as long as you want. As long as you want to hold off on finishing the main quest, there will still be some dragons appearing every once in a while. It absolutely does not mean that during your specific playthrough there would be double or triple the number of dragons you would have encountered otherwise.
Radiant Story even tracks how long its been since you encountered your last dragon, so you can be damn sure some specific dragons are going to be involved in Radiant Story quests.
Whenever you decide you want to send them back to hell or wherever they came from, you can complete the main quest and most likely they won't spawn anymore.