I hope you have yourself a super tuesday, har har har.
I do not know exactly how to answer your question. But I can offer suggestion via example. If you look at how google groups does RSS, it is a “newest reply” based feed. I am sure my example will change by the time others look at this, so I just suggest that you check out the correlation between the topics in [1], which is displayed exactly like this topic is displayed, oldest post first and a thread to the newest, versus [2], which is the exact opposite. It shows the newest replies to a post on top, so you are always getting the newest items in the feed as you subscribe to it.
As for exact numbers, maybe thats best done via configuration in the URL like [2].
[1] Redirecting to Google Groups
[2] https://groups.google.com/forum/feed/gnu.emacs.help/msgs/rss_v2_0.xml?num=10