A single site license is all you need to use the TaxoPress Pro plugin on a WordPress multisite network.
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. For that reason, 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 using the main site in your network to control and activate your license key.