Q: how to send one email to 20 addresses

By entering all the email addresses in the TO field of Mail.

You could also use the cc field if you want, or the bcc field if you don't want people getting the email addresses of other recipients.

What they said, plus you can create and use a Group in Contacts it you don't want to have to type/select 20 separate addressees every time.

See the "Using Address Book" section of this support article. It tells you how to create a Group as well as how to create a Smart Group:

Mac Basics: Address Book - Apple Support

Cheers,

GB

You are most welcome princepony! Glad you are all set now

Cheers,

GB