This is a Wordpress setting you can change. Login to Wordpress admin, click Settings > Reading, change "Syndication feed show the most recent" from the default 10 items to something higher, and click Save. If you don't see the change immediately, try closing and reopening your browser, then re-visit the /feed/json/ link.

This is a Wordpress setting you can change. Login to the WP admin panel, click Settings > Reading, change "Syndication feed show the most recent" from the default 10 items to something higher, and click Save. If you don't see the change immediately, try clearing the recent cache in your browser, or open the link in a different browser.