If you're looking to find out the maximum number of agents that are logged in at any given time, I have not found anything in CMS that would provide this information.
You can run schedule automated reports in ASA for the command: disp capacity and export the results.
This will show you the current number of logged in ACD agents (page 5 for CM 5.2.1)
It's not the greatest solution as you have to manually search through the file to see the highest number and it's still a hit or miss to get the actual highest number as you can't run this every second, but it will get you a better idea.
It also shows you your capacity under the System Limit column.
- Stinney
I love learning and passing on knowledge. "Because knowing is half the battle".... GI JOOOOOE!