Dear johnmax,
You will need to activate WP ERP every time for each subdomain of your site.
So, a single license for WP ERP add-ons will not work for all the subdomains of your site.
You can use one ERP installation on 1 database. That means one WordPress installation can have only one ERP activation. You can definitely install and activate the plugin on a single subsite of a network installation. But WP ERP cannot be used across multiple subsites of a network installation.
I hope this helps you out.
Thank you.
