If you just subscribed to a podcast and only see the last X episodes, it means that the podcast owner is just sharing those episodes via the RSS feed.
Some famous podcasts are doing this like 'This American Life' which only gives access to the latest episode. If you want to listen to older episodes you have to go to the podcast website and stream them from there. To download earlier episodes you need to buy them on iTunes.
You can easily make sure of that. To do so, long press on the podcast (not the episode), select 'Copy to clipboard' and finally paste the result in a web browser. You will see the exact number of episodes contained in the RSS feed you subcribed to.