When using the RSS Feed Builder, a user could potentially enter such a large value for the number of feed items returned that Confluence would eventually run out of memory.
When using the Feed Builder, if a users a value greater than this setting (or less than 0) they will get a validation error.
If any pre-existing feeds are set to request more than the configured maximum, they will be supplied with only the configured maximum number of items. This is done silently - there is no logging and no message is returned to the RSS reader.
If Confluence times out when responding to an RSS feed request, any items already rendered are returned.