How to choose port number for various microservices? whatever port number I use is already used-blocked or I'm not able to use them

Question

I'm learning spring boot & microservices, I have created 3 services one is running on port 9000 2nd on 8080 but while building 3rd I've tried using many other ports like 6341, 5030, 4661 and even tried using random port number but I'm encountering ECONREFUSED error in Postman

I did try listing all the LISTENING ports in my project but I'm still facing the same problem, please tell me how to determine which port should be used