Thank you so much guys... This is working now...
There is another challenge now. Above these top 50 customer names , I need another field in a row which is a count.
What have I done till now is, I have added customer name in to dimension tab and sorted it on the basis of order amount in sort tab. Expression tab has formulas on order amount. Now I need another diemsion(row above customer names) which is a count of customers. We have handled such cases before using value list but in this we need to have top 50 customer names which are not static.
Please confirm if I am able to explain problem properly. Will try to elaborate otherwise.