@mikey I think the #Dunbar number should make a good guideline for the number of people on an instance vs number of admins taking care of it.
1 admin? 150 users
2 admins? 300 users
This could be further combined with Pareto's principle (the 80-20 rule) and/or the power law to further increase the number of potential users.
1 admin? 150 power users, or 750 total users
Alternatively, if you apply the 1-9-90 principle, you could have as many as 1500 users per admin, perhaps?