上位 10 行を取得し、行 11 の他のすべてを合計する

トップ 10 のランク付け方法を指定しなかったので、最高カウントが上位にランク付けされていると想定していますか?

With TopItems As
    (
    SELECT C.CountryID AS CountryID
            , C.CountryName AS Country
            , Count(FirstName) AS Origin
            , ROW_NUMBER() OVER( ORDER BY Count(FirstName) DESC ) As Num
    FROM Users AS U
        JOIN Country AS C 
            ON C.CountryID = U.CountryOfOrgin
    GROUP BY C.CountryName, C.CountryID
    )
Select CountryId, Country, Origin
From TopItems
Where Num <= 10
Union ALL
Select 0, 'Others', Sum(Origin)
From TopItems
Where Num > 10