Ok, so. There are few things to understand:
1. When you add BCC with lots of emails, the email server will consider that you sent one (and only one) email. Why do you care about this?
2. Some hosts will not allow sending more than [x] emails per hour, where [x] may be 200 or a bit more (again, some hosts! ask at support about yours). If you need to send a newsletter, sending (example) 2000+ emails at once may get your account blocked. So, the BCC can help you a lot because you can split the number of emails to send into chunks and deliver all at once, without exceeding the limit.
3. To send emails with BBC you will have [TO]:firstname.lastname@example.org - [BCC]:email@example.com,firstname.lastname@example.org. So, do not have [TO]:george and [BCC]:mike,jhon because mike and jhon will see the email of george.
4. The best way of sending emails is to add all with [TO] and send multiple emails. If you do not exceed any limitation (as I said, talk with your hosting support) this is the best way - (as ralph.m said and also QMonkey - it's the same thing, different approaches).
Some code, from the manual with some adjustments
$to = 'email@example.com';
// also define your $subject and $message
// how to send an email with BCC
$headers = 'MIME-Version: 1.0' . "\\r\
$headers .= 'Content-type: text/html; charset=iso-8859-1' . "\\r\
// this email may not exist
$headers .= 'To: Website <firstname.lastname@example.org>' . "\\r\
// this is a good email, that must exist (some user may send a reply)
$headers .= 'From: Website <goodExistingEmail@website.com>' . "\\r\
// mails to be delivered to
$headers .= 'Bcc: email@example.com,firstname.lastname@example.org,email@example.com' . "\\r\
// Mail it
mail($to, $subject, $message, $headers);