So, yes, if you add users and want to keep them, you need to buy additional seats. My point was that you can choose to deactivate users rather than license them.
As for your second point, if you have paid subscriptions and decide to not deactivate accounts, then yes, your seat count will increase. I would contact the sales team… there are pricing models for academic, non-profit, large numbers, etc. Get a Quote | Mattermost will actually ping a Mattermost bot and whomever is responsible for your territory will get with you and you can ask them about that.