I assume that you are talking about running locally in a WA simulation environment. If so, it tries to use the port you specified (usually 80) and increments the port number one at a time until it finds a free port.
If the port continues to increase each time it starts, it means that the port is not freed when you close the application. First, make sure you actually stop the previous run (open "dev fabric" or "compute emulator" depending on the version of the SDK you are using and be sure to stop the old run). Secondly, you should be able to restore all these old ports forever if you disable the dev fabric (computational emulator). You can do this by right-clicking the icon in the system tray and turning it off, or run the command "csrun / devfabric: shutdown" from the command line.