How many people died during the building of the Hoover Dam?

The "Official Number" is 96.

The "official" number of fatalities involved in building Hoover Dam is 96. These were men who died at the dam site (classified as "industrial fatalities") from such causes as drowning, blasting, falling rocks or slides, falls from the canyon walls, being struck by heavy equipment, truck accidents, etc. Industrial fatalities do not include deaths from heat, pneumonia, heart trouble, etc.

https://www.usbr.gov/lc/hooverdam/history/essays/fatal.html

96. This number does not include those who were injured at the work site but died elsewhere.

The "official" number of fatalities involved in building Hoover Dam is 96. These were men who died at the dam site (classified as "industrial fatalities") from such causes as drowning, blasting, falling rocks or slides, falls from the canyon walls, being struck by heavy equipment, truck accidents, etc. Industrial fatalities do not include deaths from heat, pneumonia, heart trouble, etc.

https://www.usbr.gov/lc/hooverdam/history/essays/fatal.html

For a breakdown of all fatalities and their causes you might start here. It claims 111 fatalities were directly related to construction.
https://www.usbr.gov/lc/hooverdam/history/essays/fat1935.html
This essay lists 213 deaths along with the official cause; but that includes deaths not necessarily related to construction (natural causes, &etc.). The interesting number is the 42 who died from 'pneumonia'. According to a source on Wikipedia...

Workers alleged that this diagnosis was a cover for death from carbon monoxide poisoning (brought on by the use of gasoline-fueled vehicles in the diversion tunnels), and a classification used by [the construction contractor] Six Companies to avoid paying compensation claims.[80]

https://en.wikipedia.org/wiki/Hoover_Dam#Construction_deaths