For me there is no ratio (as such) between number of admins to the amount of users or content.
I would say that the more important number is 'What is the smallest number of people who need to be a site / server administrator for your organisation to function effectively?"
Having the absolute minimum of administrators is one way in ensuring the security / consistency / etc of your Tableau Environment. ie the more people who have the ability to change your server configuration, the more chance there is of unwelcome changes being made.
Therefore, if the sole reason that you have a large number of administrators is for user / group management, then I would suggest looking at ways to remove this role from those people.
Unfortunately in Tableau Server there is no concept of a 'User Admin' (Maybe one day), so you have to give enhanced rights to the people who are managing those users.
Or, you look at using AD Groups for the content permissioning instead of using local groups, or look at using scripts that leverage TABCMD or (espicially) REST API to manage those users.
As for managing content, this could be done by Project Leader Permissions, Content Owners, etc, who don't have to have the Administrator role.
Hope that this helps