The only settings that I am aware of for determining who many posts (episodes) are shown in a feed (presumably including iTunes), are in the PowerPress plugin settings. Go to your WordPress dashboard, then PowerPress / Settings / Feeds. In about the middle of that page there is a setting for "Show the most recent" and then a box to fill in the number of episodes to show. The only other setting I am aware of is in WordPress dashboard under Settings / Reading. If this isn't it, then your issue involves either iTunes itself or something truly technical, both of which I would be totally ignorant of.