You can use any port as long as it is not already in use and it is up to you to set up port allocation. It is recommended that ports are close together and sequential i.e 27015, 27016, 27017 to save confusion. Some servers ports by default are not sequential, however, there is nothing stopping you from changing this. Example for source engine game servers; you could set the port 27015, source tv port 27016, client port 27018. The next game server instance could simply follow on from this.