As I've posted elsewhere, it is one of the stretches of the novel (and film) to describe a NY mob family with just two (or even three) capos.

TB, the novel states that the big news coming out of the meeting that Michael held after he returned from Las Vegas was that Clemenza and Tessio would be permitted to form their own families from their regimes. However, that is even more of a stretch. No one Don could decide to add one or possibly two mob families to the inventory on families around the country.