Deserts are found on every continent, and they cover more than one-fifth of the Earth’s land.  To be considered a desert, a place must receive less than 25 centimeters of rain per year.
To see a map of the world’s desert regions, click on this link:
http://environment.nationalgeographic.com/environment/habitats/desert-map.html
The exact number of deserts is difficult to pinpoint, because there are some desert regions that aren’t specifically named.  Further, with global climate change, new desert regions may be appearing and others perhaps disappearng.