Installing Mountain Lion on multiple macs with one application install If I have downloaded Install OSX Mountain Lion application file from the Appstore, I can use it as an installer on multiple machines. 

Any idea for how many machines will this be valid? How apple verifies a valid install?
Five.

I wish I could give better answer, but I can't fully resolve the problem of Apple's differentiation between iTunes Store, Mac App Store, App Store and iBook Store.

If you read the following official document, Apple only talks about the iTunes Store when it comes to licensing and iTunes-authorized computers.

My understanding is that the same rules apply to all purchases made in any of these stores because the receipt via Email is always from do_not_reply@itunes.com - no matter if the purchase is an iOS App, a Mac App, or a a song...


  http://www.apple.com/legal/itunes/us/terms.html#APPS
  
  USAGE RULES
  
  (ii) You shall be authorized to use iTunes Products on five iTunes-authorized devices at any time, except for Content Rentals (see below).


Since you purchased Mountain Lion in the Mac App Store, it's an iTunes product which you can install on up to five Macs at the same time. I've installed Mountain Lion using the same self-made USB installer on three different Macs without problems.