$article = "SELECT Name, (SELECT Contact.FirstName, Contact.LastName FROM Account.Contacts)
    FROM Account
    WHERE Id IN
    (
        SELECT AccountId
        FROM Contact
        WHERE Id IN
        (
            SELECT ContactId
            FROM CampaignMember
            WHERE CampaignID = [ID]
        )
    )";

$count_one_line = mb_substr_count($article, ")"); // 3
$count_multi_line = mb_substr_count($article, "    )
    )"); // 0 ?

echo "<p>one line: {$count_one_line}</p>";
echo "<p>multi line: {$count_multi_line}</p>";