If am downloading 58 episodes approximately 400mg each how many gb is it?

I have an iPhone 5s with 64gb available 49.1gb left

At 400MB per episode, you can get approximately 2.5 episodes into each GB of space, actually a little less because there's some space used for indexing and other "overhead". The 58 episodes will need about 23.2 GB. With 49.1GB of available space, there should be no problems.