It is designed to return only the first entry.
There is no POST_COUNT but you can find the hardcoded reference to the first entry in widget() in line 71 of the file (for atom) and line 83 (for rss).
On line 89 onwards, if it has successfully read an entry, it populates $vararr[‘feedarray’][] with that first entry only.
In printcontent() on line 220, the html is generated. As it has so far assumed that every $vararr[‘feedarray’][] is it’s own blog, it will generate a new header for every index.