There are 4 DVD's all together.
They sell them singularly, 2 packs and a 4 pack box set.
I highly recommend the entire set. Of the many DVD's I've bought and rented this is by far the best of all of them.
Each DVD has 2 episodes each almost an hour long, plus special features and making of documentaries. Plus the special features and behind the scenes are actually worth watching. You will find a 5 out of 5 review on every site that carries them.
Futureshop, HMV, and A&B Sound all carry them. (Futureshop has free shipping)
The box set is usually 69.99-74.99.
My personal favorites are the Deep Sea and the Coral Reef episodes.
http://www.bbc.co.uk/nature/programmes/tv/blueplanet/