I'd like to change the username length from 2-20, please let me know which file I need to edit. Thank you!
Thank you! But my question is why I need to add. I think it should be in conf/config.php. And I just need to change the setting, but why it is not in that file, I need to add. If it is not in that file, does it mean, this code of name length setting should be any other files.
No. Vanilla can have thousands of config values coming from the core or from addons.
To spare us with an overpopulated list of values there is a config-defaults.php that features many of these values. Pick the ones you need from there, copy them over to your config.php and fit them to your forum.