Your podcast's RSS feed is managed by Seriously Simple Podcasting, however the total quantity of posts to be included in a feed is something that is set in the WordPress Admin dashboard.
1. In your WordPress Admin dashboard go to Settings (not to be confused with Podcast > Settings)
2. Select Reading
3. In the Syndication feeds to show the most recent field, set the number of episodes to be included in your podcast's feed:
By default, this number is 10, which is why it’s common for new podcasts to only see the 10 most recent episodes in their feed. Set this value to whatever you’d like, and that number of episodes will be included in your podcast feed.