How to get the last value of a certain feed

I am trying to get the latest value of a particular feed. I am using JSON to do this, I am using the following command

http://localhost/emoncms3/feed/data.json?&apikey=xxxxx&id=32&start=13174...

Which seems to work but it displays all the values, not just the last one, what does the &res variable do ? I get different results for different &res values, but I cant work out what it does.