According to the fluff, there were 20 Primarchs, for 20 Legions. What I'm asking is two-fold (if you haven't read Legion yet, I'm about to spoil the ending)

Alpahrius's twin. Omegon. Does he make it 21 Primarchs? Or do the two of them count as one?

The missing ones. What do you think happened to them? Did they refuse the Emperor, were they wiped out, did one of them run off with the Empress? What are your thoughts.

Personally I like the idea that one of them refused the Emperor and was struck down. But his followers kept him alive and took him to a secluded planet, where they rebuilt a new world and healed him. Then they found out how to make more Space Marines and now live as renegades in the Ghoul Stars. Hooray for boredom, and what wacky ideas that come out of it! Well thats my fantasy, anyone got any others, or theories, as to the missing primarchs.