Another UDF which is hopefully generic enough and does not make assumptions about whether you want to round to a specific number of decimal places:
CREATE FUNCTION [dbo].[fn_FormatNumber] (@number decimal(38,18))
RETURNS varchar(50)
BEGIN
-- remove minus sign before applying thousands seperator
DECLARE @negative bit
SET @negative = CASE WHEN @number < 0 THEN 1 ELSE 0 END
SET @number = ABS(@number)
-- add thousands seperator for every 3 digits to the left of the decimal place
DECLARE @pos int, @result varchar(50) = CAST(@number AS varchar(50))
SELECT @pos = CHARINDEX('.', @result)
WHILE @pos > 4
BEGIN
SET @result = STUFF(@result, @pos-3, 0, ',')
SELECT @pos = CHARINDEX(',', @result)
END
-- remove trailing zeros
WHILE RIGHT(@result, 1) = '0'
SET @result = LEFT(@result, LEN(@result)-1)
-- remove decimal place if not required
IF RIGHT(@result, 1) = '.'
SET @result = LEFT(@result, LEN(@result)-1)
IF @negative = 1
SET @result = '-' + @result
RETURN @result
END