I will answer your last question first, you will be able to enable more mailboxes you are licensed for and it will archive them. HOWEVER, if you ring support or have a consultant come on site and they discover you are over your licensed amount then you will need to pay for the overage.
License users refer to Active Users within the enterprise (i.e., Users actively creating new data that is being archived).You do not require a license for inactive mailboxes.
Also, this does not mean that you can buy 300 licenses and only have half your users enabled at a time. :)