There are three runner factions and four corporation factions. If you purchase 3x of the core set, you can make one starter deck for 3/3 runner factions and 3/4 corporation factions by just adding the neutral cards for each side. To have 4/4 of the corp factions it requires either a 4th core set or the ability to get all the neutral cards for the corp side.
This isn't a big deal but I was kind of hoping to have 4 "full" decks without the need to swap cards while my friends and I first start to play. As we cutomize our decks and things this isn't that big of a deal but it kind of stinks that out of the box we cannot have complete decks for every faction.
I'll be purchasing a 4th core set to get the 4/4. I can use the extra cards for new players to help them build out their collections (if they don't want a complete play set) but just thought it was intersting.