MySQL Guide to Ports
There could be from one to over ten ports used within your MySQL ecosystem. It really depends on what you have enabled, which components you are using, how your applications connect, and other characteristics of your environment.
From a security point, these ports need to be opened just wide enough to allow the various components that should communicate to talk while blocking out anything else trying to hack its way in – the goal being to enforce the security principle of “least privilege”. …
Source: MySQL Guide to Ports