On how many computers can you install the retail version of Windows 8?

(I Am Not A Lawyer.)

On one.

Under our license, we grant you the right to install and run that one copy on one computer (the licensed computer) as the operating system on a computer that you build for your personal use

and also:

Anytime you transfer the software to a new computer, you must remove the software from the prior computer.

Refer to the Personal Use License.

Full Version Retail licensing does not exist anymore like it did for XP and W7, it is replaced by OEM "System Builder" with the "Personal Use Licence" clause, essentially the same thing.

See my post here

The retail install disk only entitles you to a single installed instance of the software at any one time.

Unlike the OEM license which dies with the machine, you are allowed to reinstall on as many different machines as you want, as many times as you want - but, you must uninstall/delete it from any machine before you install it on the next.