App Store for Business
Unless you cut a special deal with Apple, you will buy one single license for each Mac and you can circulate those machines sequentially to any number of employees. The last time I checked, you would need to purchase 100 OS licenses to get a floating license agreement with Apple.

There are two simple ways to get expert advice on the licensing terms of the Mac App store:


Ask Apple - since you are in a country with Apple Retail presence you can call any of the stores and speak with a member of the Business team for advice, consultation and assistance. Next would be going to your online store and asking for pre-sales assistance. There is typically a toll free number with assistance in your native language(s) wherever Apple has an online store.
Read the license for the software. In your case, Mountain Lion would come with a license if you purchased it in bulk from Apple directly so I presume you would know in that case what to do. Otherwise, the App Store terms/conditions take first precedence and secondly the Mountain Lion terms also apply.


The Mountain Lion terms appear to be unified for all regions here:

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

You can find the App Store terms at http://www.apple.com/legal/itunes/ww/ as well as from the App Store when you select your Account Information in the footer between the Privacy Policy and FAQ.

For you, the direct link is:

http://www.apple.com/legal/itunes/de/terms.html

Most countries can substitute their two letter abbreviation for de above to get a localized and correct license for Mac App Store licenses for accounts based worldwide.