How often can we take advantage?

I'm planning a trip and will have a 19 hour stop over in Kunming (China) traveling from Nepal to South Korea, and around 3 weeks later I am going on a tour of North Korea and I am required to stay in Beijing the day before the tour group flies to Pyongyang. After the 4 night tour I will be flying back to Beijing, and then on to Japan a few hours later.
So that would be 19hours in Kunming, then South Korea, then 24 hours+ or so in Beijing, then North Korea, then another few hours in Beijing, then Japan.
Is there a limit to how many times you can enter China on a 72 hour visa in a certain time frame?