I dunno, PHB1 has: 7 Races, 3 Power Sources (Divine: 2, Martial: 4, Arcane: 2 for total of 8), Weapons, Rituals, Magic Items, and biggest of them all Rules. A large chunk of PHB1 will just be rules for playing which won't be the case in PHB2.
So you figure PHB2 will have currently:
+5 races (-2 from PHB1)
+Weapons
+Rituals
+Magic Items
-Rules
That is more space, so you figure... Say, Total of 12 classes could fit in PHB2 perhaps. We could have 3 old Power Sources, 3-4 Psionic, 3-4 Primal, and 3-4 Shadow. That would be total of around 12 to 15 classes.
Now, we don't know how few or how many each new Power Source will have, look at PHB1. So far we got basically confirmed: Druid, Barbarian and Shaman. So currently 3 for Primal. We got no numbers for the rest.