So, here is the full working code that I use to create a multi-line insert, get the added rows and the last inserted id:
$contactsTable = new Contacts_Model_Contacts();
$sql='INSERT INTO t (col1, col2, col3) VALUES (1, 2, 3), (4, 5, 6), (7, 8, 9)';
$stmt = $contactsTable->getAdapter()->prepare($sql);
$stmt->execute();
$rowsAdded=$stmt->rowCount();
$lastId=$contactsTable->getAdapter()->lastInsertId();
echo '<br>Last ID: '.$lastId;
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