I want so say hello to multiple email addresse s(20+). Is it possible to do this without showing to each people that the email was sent also to other 19 email addresses?
Yes. There are three fields that specify where email is sent.
"To" - Everyone will see this field and it is usually the primary recipient(s) of your email.
"Cc" - this field is also seen by the recipients. It usually signifies you want to send the email to these people as well, but it's not as important for them.
"Bcc" - this field is not seen by the recipients. Stands for "blind carbon copy", this field is usually used when users dont want the recipients to see other recipient's email addresses.