You just need two...
The five disc set and the special edition with the first 4 episodes on it

But be warned they don't seem to go in order on the box set so...sotry makes not that much sense if u dnt watch it in total order

Yeah, they didnt release them in order. in fact, there are 6 episodes that didnt make it to DVD.

Here in the UK, the DVDs are:

Mortal Kombat Conquest: Special Edition (4 Episodes)

Mortal Kombat Conquest: Black Dragon (2 Episodes)
Mortal Kombat Conquest: Final Battle (2 Episodes)
Mortal Kombat Conquest: Kreeya (2 Episodes)
Mortal Kombat Conquest: Quan Chi (2 Episodes)
Mortal Kombat Conquest: Queen (2 Episodes)

Theres a 3 Disc Box Set containing Black Dragon, Kreeya and Queen.
Theres a 5 Disc Box Set containing all 5.

I have 2 sets imported from Australia.
Volume 1-3 and 4-6. They contain ALL 22 episodes on them. Kinda pricy, but best DVD purchase I ever made, in my opinion.

Look for it here: http://stores.ebay.ca/DVDPoint-Australia