First of all sort your data by sales descending.
Then to create a rank field we can use the multi Row formula tool, use this to create a new field and call this RANK.
Then use the statement [row-1:RANK]+1
There is an option to group by, in this option make sure you check the week field, this means the count will reset for every week.
Final step is to use the sample tool, choose the first N and set the number to what you like.
Again, set the group by criteria to be your week field, which means you will get the Top N records for each week.