License Keys and WordPress Multisite
A single site license is all you need for WordPress multisite networks.
A WordPress multi-site network only has a single plugins directory, which is shared by all sites. So if a plugin is updated for one site, it is updated for all sites. In that sense, a single license can be used for an entire Network.
However, the sub-site which activated the license will be the only site that can check for new updates.
We recommend making a single site that controls and activates all licenses to prevent things getting unorganized.
WordPress sites installed in subdomains / subfolders
PublishPress plugin and support are provided using site licenses. That is not the same as a “domain license”, so if you have multiple sites installed across subfolders and subdomains, each site will require a unique site license.