In 3.0 Confluence will include an option to limit the number of items an RSS feed can request.

This setting can be found in 'Edit General Configuration' (see the attached screenshot) and defaults to 200.

The help text on General configuration says the following -
Maximum RSS Items:
Limit the maximum number of items an RSS Feed can request. If a feed requests more items than this setting then its requested number will be ignored and it will be limited to this value.
Default: 200

This setting is only available to a Confluence System Administrator.

When using the Feed Builder if you enter a value greater than this setting (or less than 0) you will get a validation error as shown in another screenshot attached.

Any feeds (e.g. existing feeds) which request more than the configured maximum will be supplied only with the configured maximum number of items. This is done silently - there is no logging and no message returned to the RSS reader.