You can add more packs to your license, or add more sites to a pack, by ordering a subscription upgrade.
Open your subscription (aka license) Login to your account
Open your subscription to upgrade it
Open the upgrade order form
WP Solr upgrade order form
Set a # of sites for each of your pack
WP Solr upgrade order form
Confirmation
You will receive shortly an upgrade confirmation email from Avangate, our order processor. When you do, you can connect to your gotosolr account, open your subscription, and verify that the new packs and their number of sites are updated.