question

That script does return the number of Process Flow objects in the model. However, if you want the number of process flow activities in a process flow you need to add up all the sub nodes of the process flow.

This script will add up all the activities in all the process flows in the model:

treenode processFlows = model().find("Tools/ProcessFlow");
int numActivities = 0;
for (int i = 1; i <= processFlows.subnodes.length; i++)
	numActivities += processFlows.subnodes[i].subnodes.length - 1;
	
return numActivities;