By using Index( ) in calculated filed and drag that calculated field in rows shelf we can achieve this.Please find the attached twbx.I hope this solves your problem

 

Step1: Create calc field called index ie index( )

 

Step2: Drag the customer name and calc field index into row shelf.

 

Right click customer name descending and sort via field [sales]

 

Step3: create [Top N] parameter with integer data type

 

Step 4:Create calc field called top10? Title Top 10 calculations – Top10?

 

if [Index]<=[Top N] THEN "Top N"  ELSE IF [Index]>=SIZE()-[Top N]

THEN  "Bottom n"  else “Others” end end

 

Drag calculation field Top 10? Into filter then select top n and bottom n.

 

Drag the new calculation into row.

 

show parameter control and type 10,20 etc