Since the release of EPiServer 7, CMO is included with the CMS license. You only require to install episerver 7 license (License.config)
http://world.episerver.com/Framework/Templates/Page.aspx?id=76681&epslanguage=en

For enterprise configuration, have a look at this (though its for CMO 2 but should work):

http://world.episerver.com/Documentation/Items/Installation-Instructions/EPiServer-CMO/CMO-2-R2/Configuring-Live-Monitor-for-Multiple-Bindings-in-EPiServer-CMO-20/

I know that this is sales marketing department question, but anyway good to know: if EPiServer is serving more than one site (http://world.episerver.com/Blogs/Johan-Bjornfot/Dates1/2013/12/Multisite-feature-in-EPiServer-75/) does it mean that we need license per file or it's CPU bounded?

Mark it and waiting for expert's advice. 

You will require a single license.config bound to either IP/Mac which supports multiple sites. For instance if you have two sites in an enterprise scenario then you would require a License.config which has Multisite Limit=2. A developer license has multisite limit set to 1000 as default. 

Thanks for all the answers! Really appreciate it!

I have one more question.. When downloading a License is there always a standard that the license supports multiple sites or can you download a license that doesn't support multiple sites?

All developer licenses have multiste limit set to 1000 as default and demo licenses have multi site = 1. However when ordering a commercial license (for production) you have to order a specific license based on your requirements (1 site, 5 sites or unlimited sites). 

How does it work if we have EPiServer Community and CMS for multi site? How should we configure the license file?

@Dung: 
In enterprise environment, you require one license for each CMS/Community or a license that suppots multiple sites. You can specify LicenseFilePath for <episerver.community>'s <site>-element in web.config as well just to make sure that Community license is loaded when accessing community API.