It's in the Cult Legion sections. To save time, and since it's not a quote, here's the rundown:
Emperor's Children: 6
Death Guard: 7
World Eathers: 8
Thousand Sons: 9
Not this only works with the Cult Legions above (or your own Legions/Splinter Legions that use the Cult rules)
Hope I helped,
~Necro