Since the portion from Source to Check Out is your entire system, you can use the predefined Tracked Variable, "WorkInProgress" I think the statistics that would be more helpful for you are the WorkInProgress vs Time or a histogram of the variable's changes in the system. (There are premade Dashboard widgets for this.) But if you want to get the Max or Average, you can use the "Model Documentation" widget, and add this code to the model documentation script panel:
pf(getstat(trackedvariable("WorkInProgress"),"",STAT_AVERAGE));
(Or STAT_ MAX for maximum.)
If you want to get stats on objects in a smaller portion of your model (for example the first two processors only) then you can create your own Tracked variable, and increment it when it enters that system, and decrement it when it leaves the system.
For more info on how to set up and use a Tracked Variable see this post:
https://answers.flexsim.com/questions/27025/is-there-a-way-to-display-the-average-value-of-a-l.html
I've reattached your model below with the Dashboard Widgets in place.
4015-sample-model.fsm
