As far as my personal experience has been, it is supposed to be only once per session: when you login. That should be it, that is sufficient to indicate to mac OS (regardless of what Mac you use, Touch Bar or not) that the user's identity has been verified, so Safari can autofill the usernames and passwords.
If for some reason you leave your Mac idle and the screensaver kicks in, you might have to enter your password again to resume working, but this setting can be changed (I do not recommend it - especially if others can access your machine whilst your away)
This is implied in the support article, I referenced, and I quote from it:
"Important: Safari autofills your information for anyone who uses your Mac with your user login. It also autofills your information for anyone who uses your other devices, if those devices and your Mac have iCloud Keychain set up.
For security, set up your Mac to display the login window when it starts up and to require a password after sleep or a screen saver begins. Set up your iPhone, iPad, and iPod touch to require a passcode to unlock the screen.
Tip: If you use iCloud Keychain to keep information safe on your Mac and other devices, you can use AutoFill to enter previously saved user names and passwords on all those devices."