Rss feeds limits 5 feeds

Hi, im trying to use rss feeds, but each time i do it i get only 5 items (this only happens with rss of my own site, with other sites it works good).
For example, i set a widget with the rss of my forum to get the last replies, and i only get 5. Other example would be with mailchimp campaigns, the same thing happen..
any idea?
thanks!