ES/SHM_USER_COUNT is a SAP Parameter attribute which is used to control Number of ES users to support information. This is available within R/3 SAP systems depending on the version and release level.
Below is the standard documentation available and a few details of the attributes values .
This parameter describes the maximum number of users in all open
sessions. Default is 1024. Do not set this to be higher than 8000!
See SAP Parameter Documentation for full SAP documentation for this profile parameter.