MailMessage msg = new MailMessage();
msg.Body = ....;
msg.To.Add(...);
msg.To.Add(...);
SmtpClient smtp = new SmtpClient();
smtp.Send(msg);
To
is a MailAddressCollection
, so you can add how many addresses you need.
If you need a display name, try this:
MailAddress to = new MailAddress(
String.Format("{0} <{1}>",display_name, address));