The Pro account is per Trello team. All the PRO features are automatically available on all the boards of the team you chose when you registered for the trial.
Elegantt PRO is available for the Trello Team you chose which means you'll be able to use Elegantt PRO on that same team's boards.
If a Trello Board is not from the Team you registered to Elegantt PRO with, then you will have the free Elegantt version.
You can access your account at any time to see how many members of your team are actively using Elegantt. That way, you can easily manage slots and decide to pay only for the members of your Trello organization that are using Elegantt.
Members and slots are managed separately, so you can remove a member from a slot at any time to make space for a new member on your PRO plan.
If all the slots are already taken, a new slot is created, automatically for monthly plans, to let the new member take advantage of PRO features.