It depends on what configuration you want for your pool. Do you want a RAIDZ1 or RAIDZ2 and how much storage space do you want. Reworded, how safe do you want your data and double the storage space that you think you need.
Here is a link to a RAID calculator
http://www.servethehome.com/raid-calculator/
In my opinion if your are using the NAS for computer backups and a media server, use a RAIDZ1 configuration.
For a RAIDZ1 with four 2TB drives you have 5.5TB of usable storage.
For a RAIDZ1 with five 2TB drives you have 7.3TB of usable storage.
If you are going to store important data on your NAS and you would rather not ever desire to reinstall your data then having a 2 drive fault tolerance is more important so a RAIDZ2 makes more sense to me. This is what most people eventually run.
For a RAIDZ2 with five 2TB drives you would have 5.5TB of usable storage.
So first figure out how much storage you need and the protection level.
For myself I have six 2TB drives in a RAIDZ2 configuration. All my drives have a 3 year warranty so I'm good for drives until late 2015 and early 2016. I will buy a spare 2TB drive a few months before my first drive goes out of warranty. I do not have a spare 2TB driver on hand because I have no problems submitting an RMA where the replacement drive is shipped immediately and when I get that drive, I return the failed drive. You have to give up your credit card info but that is fine by me. And I should be able to run my system until the new drive arrives. If I have a second drive failure before then, well I'll shut the system down and wait for the drive to arrive. And I always maintain a separate backup of all important data. Other data like movies, well if I loose them I know I won't be happy because it took a long time to rip them but I won't loose sleep over it either.
I hope this helps.