Ritu,
The Interactive Web Services API Documentation at, https://rally1.rallydev.com/slm/doc/webservice , has further information. For this specific question, these two sections apply:
pagesize
Number of results to display on the page. Must be greater than 0 and less than or equal to 200 for 1.x API calls or 2000 for 2.x API calls. The default is 20.
Example: pagesize=30
start
The start index for queries, which begins at 1. The default is 1.
Example: start=1
Since you have more than 2000 records to fetch, you will need to make multiple calls incrementing the "start" field as appropriate. In the Web Services API return there is a "TotalResultCount" field that comes back, you will know how many Total Records there are to fetch.
So in your case, the first call would be:
https://rally1.rallydev.com/slm/webservice/v2.0/releasecumulativeflowdata?workspace=https://rally1.rallydev.com/slm/webservice/v2.0/workspace/46772661387&query=&fetch=true&pagesize=2000&start=1
And the second would be:
https://rally1.rallydev.com/slm/webservice/v2.0/releasecumulativeflowdata?workspace=https://rally1.rallydev.com/slm/webservice/v2.0/workspace/46772661387&query=&fetch=true&pagesize=2000&start=2001
And so on.
Hope that helps.
Michael