HI Kishore
that's actually not a great answer as
a) it slows the app with greater data load
b) is much too much for users as Masa has indicated
C) is a modification (I believe as I can't recall that in the extensibility options)
What's actually wrong here is not the number of items retrieved by default - after all they will appear as the user scrolls past the bottom of the list. The problem is the workitem count shown at the top of the master list. The count should show the total number of items in the current filter.
Now that is usually a bug in the OData service logic & I actually recall seeing a couple of SAP Notes for Fiori My Inbox already correcting the count.
AS you have not given details of your release/sp combinationfor Gateway or My Inbox I cannot confirm which note would fix your problem.
However a search of the Notes database on support.sap.com would be a good idea & if you can't find the answer suggest you raise a SAP Message
rgds
Jocelyn