Podcast block only shows one post regardless of "number of items" setting (Simple sites only)

### Steps to reproduce the behavior

1. Insert a Podcast Block (Jetpack)
2. Paste a RSS audio feed (http://juliagomez.es/category/audio/ in my testing)
3. Try changing the number of podcast episodes to display

#### What I expected to happen
I expected it to change the number of episodes being listed, the way it does on Jetpack Sites:

#### What actually happened

Only the latest episode displays, regardless of the number of number in the block settings.


As I said before, this issue is **happening on simple sites only**.

Here are two examples:

https://juliagomez.es/podcast/ (Feed: http://juliagomez.es/category/audio/ )
https://claragomez.es/podcast/ ( Feed: http://claragomez.es/category/audio/ )

Start by reproducing the Podcast Block on a Simple site with the RSS feeds from the issue, then trace the block's handling of the number-of-items setting and its rendering path. Done means the selected number of podcast episodes is displayed instead of only the latest episode, while Jetpack Sites behavior remains unchanged.

Written by the indexing model from the issue text.