Please remember that this is a rumor. Take it with a good amount of salt.
Have heard from a closely linked to FW little birdy, 20+ new units will appear. So I'd guess we're looking at something for each legion + power armour/cataphracti custodes is a good basis to work from.
The question does remain, what unit types or add-on kits would you like to see for the Horus Heresy books.?