Well you don't specify where you want to get this number from.
Since you said generate I'll assume you're creating it on the spot. So maybe add onto the end of $email_subject
something like $rand(100,1000)
?
Maybe you should clarify what you're asking for?
Side note:
Consider using filter_var($email_a, FILTER_VALIDATE_EMAIL)
to check emails, not some preg_match. More info on that here.