How to perform a multiplication by a different select query
Probably simplest would be:

DECLARE @var VARCHAR(7), @c INT;

SET @var = 'xtpo';

SELECT @c = COUNT(DISTINCT PcbID) FROM dbo.tbl_C WHERE dbo.tbl_C.field_5 LIKE @var;

SELECT 
    row_number() over(order by dbo.tbl_A.field_1) AS [1st column], 
    dbo.tbl_A.field_2 AS [2nd column],
    dbo.tbl_B.field_1 AS [3rd column], 
    dbo.tbl_A.field_3 AS [4th column], 
    COUNT(temp0.reference) AS Qty,
    COUNT(temp0.reference) * @c AS [6th column]
FROM dbo.tbl_C tbl_c
...


Note that I changed your 'single quote' aliases to use [square brackets] instead. Single quote aliases are deprecated in some forms, and they are not well-received because they make your aliases look like string literals. In most cases you are better off not using delimiters at all if you don't need them (which means not using column names that require delimiters in the first place).