It does not work on all cells at once. For a cell's genes to be edited, the cell must receive its own copy of CRISPR-Cas9 or whatever other gene editing system.

Delivery of gene editing systems to the right cells is a major challenge. At present, most gene editing therapies involve:

* extracting blood cells from a patient, editing them ex vivo, optionally selecting for successfully edited cells, and readministering to patients

* delivering gene therapy with viral vectors and treating diseases where you don't need to edit 100% of relevant cells for the treatment to be effective, and targeting cells that are easier for the viral vectors to infect (typically motor neurons, the eye, blood cells or the liver)

It isn't really possible to edit every or most cells in the body with current technology. It is possible to edit gametes but it is not currently ethical to do so as we don't understand the risks and the potential for abuse is huge