Is there a limit to how many machine's i can install one copy of mountain lion?

you can install it on as many computers as YOU own.

here is what Apple has to say:

"

B. 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;

(ii) If you are a commercial enterprise or educational institution, to download, install, use and

run one (1) copy of the Apple Software for use either: (a) by a single individual on each of the

Mac Computer(s) that you own or control, or (b) by multiple individuals on a single shared Mac

Computer that you own or control. For example, a single employee may use the Apple

Software on both the employee’s desktop Mac Computer and laptop Mac Computer, or

multiple students may serially use the Apple Software on a single Mac Computer located at a

resource center or library; and

(iii) to install, use and run up to two (2) additional copies or instances of the Apple Software

within virtual operating system environments on each Mac Computer you own or control that is

already running the Apple Software, for purposes of: (a) software development; (b) testing

during software development; (c) using OS X Server; or (d) personal, non-commercial use.

The grant set forth in Section 2B(iii) above does not permit you to use the virtualized copies or

instances of the Apple Software in connection with service bureau, time-sharing, terminal sharing or

other similar types of services."

http://images.apple.com/legal/sla/docs/OSX108.pdf