1) You can send individually like you mentioned (with 20 recipients, it's not bad as long as it doesn't happen regularly)
2) You can create a mailing list using a mail client. I don't believe gmail has this option. But either way, it won't be personalized. Either the recipient will just see the group name/address or they will see all the recipient's email addresses. It depends on how it's set up and what the mail client can do.
2a) Try looking into Google Groups to see if that's an option for you.
3) A mailing list manager / web application, such as PHPlist or DADA Mail.
4) A mailing list service, such as Aweber or 12all.
bccfield will help you. – whitequark Aug 23 '10 at 21:17