Hi there! When I was in halls, my room number was along the lines of 'A123' which meant Block A, Floor 1, Flat 2, Room 3. This might vary between different halls so I'd recommend checking with accommodation staff when you arrive or via email or checking to find out on the student portal if the information is available on there for your specific halls.
Best of luck and enjoy your new halls of residence! 🙂
-Alfie, University of Plymouth Undergraduate Student Rep