In General Pricing FAQs, you can read about the differences between roles in Coda. The main takeaway for "maker billing" is that only Doc Makers are charged. Admins are a sub-category of Doc Maker with the ability to modify the workspace settings and permissions.
So, to recap, Admins are a form of Doc Maker, and these two roles are the only roles you pay for. In addition to this, you can have unlimited editors and viewers for free.
Within your Workspace, you're able to view the the roles of each of your members before you upgrade. As an Admin, you can change the permissions for each user.