I have a problem I cannot separate the array by commas if I put the implode it only shows the first number and not the second in database i need insert in database 500,756

global $wpdb;  $arg=array(500,756);

$var = implode(",", $arg);   --> return in db 500 ->I can not separate by commas, num 500 is only show in db 
$var = implode($arg);        --> if i put this instead of the other return in db shows 500756

     $wpdb->insert('number',array( 
          'num' => $var,   
        ));