Quick answer
The official game page describes four core factions, while its faction area presents five selectable profiles: Orks, Adeptus Mechanicus, Necron, Blood Ravens, and Dark Angels. Blood Ravens and Dark Angels are separate Space Marine Chapters that share a broader archetype but have their own profile pages.
Faction profiles
Core factions
Orks, Adeptus Mechanicus, Necrons, and Space Marines make up the four core faction archetypes presented by the official site.
Space Marine Chapters
Blood Ravens and Dark Angels are represented as separate profiles. The supplied Dark Angels material describes them as sharing core Space Marine strategies and equipment while retaining chapter-specific style, abilities, and commanders.
FAQ
Why does the site show five profiles?
The extra profile comes from presenting Blood Ravens and Dark Angels separately within the Space Marine archetype.
Is Adeptus Mechanicus new to the series?
The supplied reference material identifies Adeptus Mechanicus as making its Dawn of War series debut.