Archiving articles means storing the links of articles. Here you can limit the number of articles stored and the method used for archiving. These settings are global settings, used by all feeds within Akregator. If you want to use a custom setting for a feed, you can set it in each feed properties dialog in the archive tab.

The Archive tab
Default Archive Settings
Keep all articles

All articles are kept forever.

Limit feed archive size to:

If the number of articles exceeds the chosen limit, the oldest articles are deleted. Note that flagged articles are ignored when counting the number of articles: if your limit is 500, and you have 510 unflagged and 50 flagged articles, Akregator will ignore the 50 flagged ones and only delete the 10 oldest unflagged articles. So in this example, 550 articles would be kept.

Delete articles older than:

Articles older than the specified number of days are deleted from the archive, unless they have the keep flag set. Akregator checks for expired articles at startup and then once per hour, so expiry may be delayed.

Disable archiving

No articles are stored - all articles are discarded when quitting Akregator.

Do not expire important articles

Right clicking on an article opens a context menu where you can mark this article as Important. Articles marked as Important will not expire, they will be kept.