In order to remember because it was hard to find, here is the simple way to set up a Mastodon Instance as Single-User-Mode, or the opposite, open to the public an instance that is set in single-user-mode
First, go to the installation folder, usually:
/home/mastodon/live/Once there edit the file .env.production file and look for the variable SINGLE_USER_MODE and set it to true or false according to your needs.