@ahaldorsen From 'about the plans' in the pricing page: "The number of registered users on your server is not limited. There is also no limit on the number of active users. This value is just provided to give a rough estimate based on usage seen across Mastodon servers." - https://masto.host/pricing/
I really don't know of a better way to provide an illustration on what plan someone should choose when you are first getting started and don't have an idea on your resource needs.