As you know, Quick Fields Agent 8 actually has an option for parallel processing, which means it can be set to run more than one session simultaneously. Performance can vary based on the types of sessions to be run in parallel. If only one of the sessions is resource intensive, they should both perform well with parallel processing, but with more than one session involving resource-intensive processes such as OCR, the performance of both may be reduced. To directly answer your question, for each installation of Quick Fields, a maximum of two sessions can run per CPU on the machine running the Quick Fields Agent Service.
If performance is an issue, Quick Fields Agent can easily be configured to schedule each session to run at different times. When you create a schedule, the session will run at the designated time, except in the case that another session is already running. By default, the next session will become queued: it will be next in line and will run as soon as the previous session finishes. There is even an option to set a scheduled session’s priority such that if multiple queued sessions become delayed beyond their designated start time, the Quick Fields Agent can prioritize based on importance.
Hope this helps.