Your Manifestly subscription is only for the number of people you add to Manifestly, not the number of people in your Slack account.
When you connect your Slack account to Manifestly, you have the ability to easily add Slack users to your Manifestly account. This doesn't happen automatically though, which means you control how many of your Slack users end up being included in your Manifestly subscription.
To manage access to Manifestly, go to the Team Management tab. You'll notice an edit button for every department you create.
And in there you'll see the ability to set a Slack user's role in Manifestly. If you give them the role of "none", then they are not in the Manifestly app and are not included in the number of users you'll need to pay for with your Manifestly subscription. So while you might have dozens or hundreds of people in your Slack account, you might then have only a subset of those people in Manifestly for your subscription.