WPSSO Pro is licensed per WordPress Site Address URL, and each purchase (of one or more licenses) is assigned a unique Authentication ID number. This Authentication ID will allow you to install, upgrade and activate the Pro version on one or more websites, up to the number of licenses you purchased.
Discounts are available for purchases of 2 or more licenses. The 2 license pack is a good choice if you have both a Staging and Production website (as offered by WP Engine, for example). Websites that are load-balanced across several servers only need one license, if they use the same WordPress Site Address (URL) on all the load-balanced sites.
Sites within a Network / multisite use different WordPress Site Addresses, so each site (aka blog) wanting to use the Pro version features must be licensed. The Open Graph+ settings in the Network admin interface allow you to define a default Authentication ID for all sites (which can be customized / modified in each site's Licenses settings page), or force all sites to use the same Authentication ID. Take note that the default site must be licensed as well, to allow Pro version updates from the Network Admin interface - so purchase a number of licenses accordingly.
As an example, if you purchase 5 licenses, you will receive a single Authentication ID, which you can re-use for 5 websites (aka WordPress Site Address URLs). You can also get 5 individual Authentication IDs, all different, by purchasing a single license 5 times.
As a service to our clients, we offer the option of merging 2 or more Authentication IDs under a single Authentication ID number (to ease license management issues). If you would like to merge 2 or more Authentication IDs, open a new support ticket and include all Authentication IDs to be merged in your request, along with the one Authentication ID you would like to keep.