In mailing all recipients are not showing
Indeed, civicrm automatically excludes from the mailing recipients contacts that:


have opted out (do not contact me)
have bounced
have said they do not want to receive emails


(I might miss a criteria or two)

Check some of your "vanished" contacts, the contact summary page should make clear why she isn't included.

Bounce emails is a common issue. It might be that you had a technical problem once and that your mail server wasn't willing to deliver the emails (eg. because you have a quota on how many emails you can send per hour) and were incorrectly flagged as bounce.