Heres another way, but it assumes that you dont care if other Rows are duplicated, and will add rows to New_Table Provided the total in New_Table doesnt exceed 3 rows

With Cte AS (

Select

ID

,National_code

, ROW_NUMBER() OVER (Partition by National_Code Order by Id) Rn

From #OLD_TABLE

)

INSERT INTO #New_Table

([ID],National_Code)

SELECT [ID],c.[National_Code]

FROM Cte c

left JOIN (Select National_Code

,Count(*) e_cnt

From #New_Table

Group by National_Code) chk

on chk.National_Code=c.National_Code

Where Rn+ISnull(e_cnt,0)<=3