I say 2. Given that we know we will have the Base Four: Cleric, Fighter, Rogue, Mage; giving 2 subclasses to each will result in a nice even dozen classes for the initial release.
Personally, I wouldn't mind 3 each, but then you're looking at 16 classes to start and my sweet spot has always been 10-15. Not so few that you feel you don't have options, not so many that you are overwhelmed with possibility.
NOW, adding in to that, what you were saying about domains, traditions, pacts, heritage-s(?), etc. etc. I can eeeeasily see the capacity within those initial 12 classes to generate at least twice that many archetypes for characters.
So, I'd go with somethin' like-a dis...
Cleric: Domains and theme/specialties allowing for any number of diverse kinds of clerics and priests.
Sub-class 1--Druid: Specialties and/or whatever the druid "domains" will be ("circles" or "groves" or who knows what) that can make 3 different druidic types: the shapeshifter, the animal companion guru...and probably a "summoner" since so many people seem to like that flavor...can't stand it myself. But *shrug* different strokes n' all that.
Sub-class 2--Shaman: Specialties and whatnot allow for...I dunno, 3 variants: Spirit communing/summoning girl, Rune/bone reader diviner, n' something else.
Fighter: Styles and specialties allow for all kinds of variations from the swashbuckler (probably also doable through a Rogue class base) to the cavalier knight. The Barbarian (berserker/battlerager guy) and Warlord go here. They are Fighters with their bells and whistles tacked on through theme/specialty and fighting style choices.
Subclass 1--Ranger: I would try to keep the variants to 3 but I know you could do several more with minimal use of specialties and backgrounds. I don't rangers as needing their own "domain/tradition/scheme" type addition. But there will probably be one anyway.
Subclass 2 --Paladin: again 3 variants for beginning play. You can get into any number of more in later books. I WOULD suggest, and it's not a themes/backgrounds thing, really, that an "Optional Module" for Paladins be included from the get go to allow for those who want 1) Using various alignment restrictions v. those not wanting to use alignment at all; 2) those who want the Paladin to be based on "Ideals" or "Virtues" versus a deity/temple.
Mage: we already know how this is gonna go. Specialties will supply the specialist magics.
Subclass 1 --Sorcerer: I would start with only 3 heritage options: Dragon, Elemental, and...somethin' else but not the same thing that Warlocks get.
Subclass 2 --Warlock: 3 Pact options to start, if i had my way they would be (in order of liklihood): Devil, Daemon, Demon. But I suspect it will look more like: Devil or Demon, Fey, Star.
Rogue: like clerics and fighters, schemes and specialties and backgrounds will allow for a HUGE number of various roguish archetypes, including as we've already seen the "Thief", no doubt some that can adequately simulate a ranger or swashbuckler, non-magic bards, pirates, bounty hunters, acrobats, jesters and jugglers, and who knows what else.
Subclass 1 --Assassin: specialties that allow for...again, let's say 3 variants including the much beloved shadow-magic assassin and I dunno what else but hopefully the default is non-magical.
Subclass 2 --Bard: start with specialties or "colleges/bard schemes" for 3 variants: the Skald (heavy warrior-bard), the Loremaster (heavy [preferably druid] magic bard), and the Wanderer (heavy exploration skill and knowledge bard).
There's more character concepts in there than I can shake a stick at. And I am a man who knows how to shake a stick.
If you want to get really technical and don't mind a bit of re-flavoring, stick the Monk in where the Shaman is (I have no problem and always consider the Monk to be a cleric-ish divine-based class) and you've gotten in just about everyone [who appeared in a 1st PHB] without having to make exception.
For those who want to see a Psion out of the gate...which I wouldn't mind either...but, sorry. Gotta respect the Sweet spot.
Psion makes it a baker's dozen and doesn't fall in line with the formatting proposed (just now, in this post, by me I mean
). I am not including peripheral shoehorned specialized specialty specialists just to get 3 psionic classes to start either...I looove my telepaths and tekes, though.
I WOULD thoroughly like a "Psychic" or "Fortuneteller" or "Medium" or something like that specialty or background for mage classes to give them a liiiiittle psionic sumin' sumin'...or, as some have suggested/hoped for, ye olde Wild Talent as something anyone can take.
But there you have it: 4 Base Classes: 2 sub-classes each: with special variants not exceeding 3 each.
...and proposed guidelines to direct players to replicate those classes that do not have their own "class" anymore:
Fighter with a Barbarian or "Savage" background and a Berserker specialty (or fighting style) = the "traditional Barbarian class"; Fighter with a Noble or Veteran background and "Warlord" specialty, etc...
Happy Saturday [EDIT: that's "Happy Sunday", now, since the site was all wonky when I ried to post this yesterday /EDIT], all. Carry on.
--Steel Dragons