53 is probably sufficient, but if a leap year starts on Saturday then Sunday Dec 31 will be in week 54.  Hopefully they don't mind missing Sunday sales on New Year's Eve once every 28 years.  This happens in 1916, 1944, 1972, 2000, 2028.

You could merge that day into week 53:

SELECT   SUM(TotalSalePrice) AS SalesForTheWeek
        ,CASE w.WeekNo WHEN 54 THEN 53 ELSE w.WeekNo END AS WeekNo
FROM    Data.Sales
CROSS APPLY ( SELECT WeekNo = DatePart(wk, SaleDate) ) w
WHERE   YEAR(SaleDate) = 2016
GROUP BY  CASE w.WeekNo WHEN 54 THEN 53 ELSE w.WeekNo END