The numbers given were always "expandable to 70, then 80 thousand seats". Also, the seats are to be added as a third deck on the north and east sides. No seats will be added to close the horseshoe. Essentially, the seating will make the whole stadium the same height as the home (south) side is now.
If they will add 20, and then 10 thousand seats, I see them adding the north side (20K) first, and then the 10K behind the student section at a later time.
Those were always the official numbers given, but the idea that they could add just 10K or even 5K at a time to what we have now, has never been confirmed... even though the concept has been thrown around by fans.