If you subscribe to Reviewable for private repositories, you can now directly limit the users who will count against your plan's quota:
If you configure this option, reviews will only be created for pull requests authored by members of the given team. (This holds true whether the review creation attempt is due to a repo connection or direct navigation via the review dashboard.) So if you're on a 10 contributor plan, and constrain your subscription to a team with no more than 10 members, you'll be sure to never exceed the quota.
Previously, you were able to limit the power to create reviews to a team, but this proved confusing since a single person on this team could connect a repo that would cause the creation of reviews with any number of contributors. This legacy option is no longer available but if your subscription was configured this way the restriction will continue to be enforced until the next time you edit the subscription (for any reason).
Hopefully this new option will be helpful and easier to use — let me know either way!