I think there is an ALERT : PEGA0030 which gives alert each time the threshold exceeds the defined value. By default, the alert is enabled with a threshold of 200 requestors. You can modify the setting in the prconfig.xml file as follows:
Open the prconfig.xml file and update this alerts section:
Thank you for your response. This setting is only for alerts. This does not control actual number of sessions or requestors. Do you know is there any setting controlling actual number of sessions or reqeustors?