in site configuration there is field for site email.
Is it only one email allowed?
Yes, since the sender of an email can only be one address (AFAIK).
Yes, it only allows a single email address. Some programs allow you to add more than one by using a ; or a , between the addresses, but Drupal checks and invalidates the field if you try that.
While you can only use one email in the box, you can set up an email alias/group/list and add separate users to the group. Then plan the group email address in your site info.
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