Thanks to the answer of @"Alex Mitchell", I realized that the
alt_changes_tab setting is responsible for the behavior I would like to modify, and I realized that this setting can be accessed and changed through dconf-editor, so I eventually solved my problem.
I mean, changed in the sense that the Alt+num keybindings can be completely disabled, not configured, but this is enough for me.
To answer to the comment:
install the package
dconf-editor, I did it from command line
sudo apt-get install dconf-editor
launch the application, it is called Editor dconf,
- search for the string terminal, clicking on the search tool icon on the upper right part of the window
- click on /io/elementary/terminal
- click on settings
- toggle the switch on the right of the setting on alt-changes-tab