User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-3146930-20121108002440/@comment-3146930-20121109032125

Exactly, all the other books have only had 2. However, the first book had one defeated and one still active giant. Meaning that if we have 2 more giants in the next book, we may also see Porphyrion as well, bringing the total to 3. I also think that there may be one giant at each end of the Doors of Death, plus Porphyrion.