How to monitor number of concurrent authors?

http://localhost:4502/system/console/tokenmgr

You can try below commands
To see the number of authors that have used the system since installation use the command line:
cd <cq-installation-dir>/crx-quickstart/logs

Command:
cut -d " " -f 3 access.log | sort -u | wc -l

To see the number of authors working on a given date:
grep "<date>" access.log | cut -d " " -f 3 | sort -u | wc -l