You can use the SQL Server configuration manager to check which port is used by any of the instances that you have on the server. Notice that the default behavior is to use dynamic port allocation (e.g. each time that the service starts, it will use a different port). Since you need to enable work on the ports in the firewall, you should modify this behavior and assign a permanent port for each instance. You can do that from the SQL Server configuration manager.

Adi