I suggest using 'Quote Chain' service instead of RetrieveItem_3 for getting constituents information for indices. You can just get the list of constituents with RIC codes as well as data for all the constituents. The GetChainComponents_1 service also contains a flag 'instrumentcount' indicating the total number of records in the response. The request also allows you to paginate the response. So for index chains like S&P 500 <0#.SPX> you can make multiple requests by changing page number and get the total no. of records that are part of the index. I have attached sample request and response.