A session will be generated whenever a user views a workbook in browser using Excel Services. The session settings for excel services can be configured in SSP. To configure these settings, navigate to SSP site and click on “Edit Excel Services Settings” from “Excel Services Settings” section.
The default session per user is 25. You can even use “STSADM” command line utility to configure the Excel Services Session management setting.
STSADM.EXE –o set-ecssessionmanagement –ssp mysharedservice
This will increase the session per user to 40.