Yes. You may volunteer in as many countries as you wish. The US$275 application fee is a one off and covers you for as many programs or countries of your choosing within a 12-week period.
When applying, there is a dropdown section for any additional projects/ countries you wish to take part in. This allows you to select a second project, either in the same country or a different destination.  If you would like to volunteer on 3 or more projects, just write this down in the ‘extra information’ box and you can talk through your itinerary with our volunteer manager.