Review the "Sensor Statistics" and "License Information" panels.
Online Sensor Count: the number of active endpoints right now. (Last 5 minutes)
Total Sensor Count: the number of all total endpoint records in the database (all time). This number is not used by licensing.
Current Sensor Count: (License related) the number of endpoints that have been active in the last 24 hours. These are the only ones counted for license purposes.
It's normal and expected to see older, outdated sensors (even older than 90 days) to appear in the web console.
Additional Information
The "Online Sensor Count" is the total number of sensors reported in the last 5 minutes.
The "Total Sensor Count" is the sum count of all sensors ever installed - every time a new sensor registers with the cluster, EDR increments this number. If a sensor is uninstalled and later reinstalled, then it counts as 2 for this value. This value has no licensing effect.
If the same license is used on multiple EDR servers, the combined license count won't display on the server console, only the individual license count being used per server.