For the activities section of the Common App, should volunteer work be listed as one activity or should I denote each institution that I have volunteered at as a separate activity?
Nothing wrong with consolidating stuff. Whatever works for you – as long as you get the info across.
You can do it either way. If you feel you are light on activities, make each one an activity. If not, create an activity called Volunteer, put “See Additional Information” in the description. Then in Additional Info you make a header called Volunteer Activities, and concisely bullet out your activities underneath.