How can I update Task % complete based on the number of Resources assigned to the Task and their actual Resource Status?

You can use the following script to get the planned count of each Resource assigned to the Task -
TASK(SELECTED) SET_PROPERTY(PHYSICALVOL_SH, "Item", COUNT(RESOURCE))

And then use this other script to update the actual count based on the status -
TASK(STARTED OR FINISHED) SET_PROPERTY(PHYSICALVOL_AC, COUNT(RESOURCE(STATUS_NAME("Default category")=="Erected")))

If the Task Progress Type is set to Physical, this will automatically update the % complete.