Is it true that every human has a cancer cells in their body ?

Sort of: Cancer cells begin as normal cells. Since our body is always replacing old cells with new one, sometimes mutations occur during this replacement process. Most mutations are picked up by our body and destroyed and a new cell is made to replace the old. Rarely mutations occur that are not detected by our body and allow those cells to become immortal and make more of themselves. That is cancer.

The story that: Our bodies may develop genetic changes that if not checked could lead to overt cancer, but the surveillance role of the immune system, when working optimally, can identify and destroy. But i'd not endorse the idea that "every human" has viable cancer cells in their body.