You can control the RSS output for Syndication in this section. The maximum days in RSS setting defines the maximum number of days that your RSS stream shall contain. This, again, is not a “number of calendar days” value, but a cumulative value. If you have posted only on 15 days in the last year and this value is set to 10, the RSS will contain the most recent 10 days of those 15.

The maximum entries in RSS value is a number defining the overall maximum number of entries that can be contained in your RSS stream. If the number of entries gathered within the days restricted by the maximum days in RSS setting exceeds this number, the only the topmost, more recent, entries, up to the count specified by this value are rendered into the RSS stream.