Does increasing timeout increase your number of concurrent sessions?
This question is regarding Service Desk classic web interface.
Release : 17.3
Increasing timeout won't increase the number of concurrent sessions, if the end users log out after usage.
However, if the end users simply close the browser instead of log out, then yes most likely you will see some bigger number in concurrent sessions as it will take longer time for Service Desk to time out those "orphaned" sessions.
Bottom line, end users should log out instead of simply closing the browser.