Before I answer your query, I want you to understand the role of a supervisor in your organization based on requirements or KPA and KPI. An individual taking charge as a supervisor should have the ability to handle their team, understand their team members, and guide them towards the right path.
A "supervisor" means not just supervising to what extent individuals are carrying out or delivering their responsibilities at the workplace within the stipulated timeframe. It's something more than supervising; you have to find out supervisory skills in your supervisors.
Based on my views, you are requested to define the ratio and please try to avoid assigning excess workload to any of your supervisors beyond their capacity, which may slow down productivity.
With profound regards,