How many copies of Mountain Lion does one App Store download get me?
You can install Mountain Lion on several Macs, provided that you use the same Apple ID for downloading it on them.

The Mountain Lion License Agreement explicitly says:


  Mac App Store License. If you obtained a license for the Apple Software from the Mac App Store, then subject to the terms and conditions of this License and as permitted by the Mac App Store Usage Rules set forth in the App Store Terms and Conditions (http://www.apple.com/legal/itunes/ww/) (“Usage Rules”), you are granted a limited, non-transferable, non-exclusive license:
  (i) to download, install, use, and run for personal, non-commercial use, one (1) copy of the Apple Software directly on each Apple-branded computer running OS X Lion or OS X Snow Leopard (“Mac computer”) that you own or control.


You can read the license by clicking the Terms and Conditions link at the bottom of the App Store window (left of the FAQ currently) and searching the license for the words "product usage" (presuming your store's terms are in english).