We highly recommend you enforce 2FA for all users.
In order to enforce 2FA, you will need to be an administrator who has already previously setup 2FA. See Setting Up Two Factor Authentication.
As an administrator, navigate to Admin -> General.
Then, click on Force all users to have two-factor auth (2FA) enabled. Hit Update.
Now, all users will have to enable 2FA the first time they login to their account.