Yes. Single trip travel insurance can cover an itinerary that includes more than one country, if it is one continuous trip from when you leave home to when you return home, and all the countries you visit are within the destination region you selected.
However, it's important to correctly state all of the countries you are travelling to when getting your quote - If you need to make a change to your destinations - See here
You can visit up to 10 countries on a single trip. If you are travelling to more than 10 countries, you will need to speak to an adviser.