Why are so many people named "Chavez" ("keys" in old Spanish)? What kind of keys?

Chavez is a variation of the less common name Chaves, of Portuguese origin. It is either an occupational surname meaning "key maker" or it is a regional surname meaning "from Chaves," which was the name of several areas of Portugal and Spain. It can be traced to the Galician region of northern Spain, and there were people with the last name Chavez in the New World as early as the 16th century.

The word Chaves comes from the Portuguese "chaves," meaning "keys," which derives from the Latin "clavis." The Spanish word for keys is either "llaves" or "claves."

https://en.wikipedia.org/wiki/Chávez_(surname)

It can be either an occupational or geographical surname, which probably explains why it is a fairly common name. It is not one of the most common Hispanic surnames, however. It actually ranks 22nd.

http://genealogy.about.com/od/surname_meaning/p/chavez.htm