I think the answer is ZERO.
I don't think any of the classes outside of the standard four will be subclasses of anything.
What does being a "subclass" even mean? Seems to me that unless it shares some mechanics of the 'base class', being a "subclass" is nothing more than a categorization. But what would be the point?
I do think though that we'll see a large number of 'builds' (for lack of a better term) for ALL the classes that get included in the game. So we'll see probably 6+ Fighting Styles, 6+ Schemes, 10+ Domains (one for every Greater God that is included as an example in the book, whether that be the Realms gods or the 4E pantheon) 4+ Traditions (or even more if the Spell School Specializations become Traditions in addition to being Specialties), 3+ Patrons/Pacts, 3+ Sorcerous Origins, 3+ Ranger Organizations, 3+ Paladin Codes, 3+ Druidic Sects or Wildshapes (or whatever differentiation they use to make druids different) etc. etc. etc.
Here's the thing... I think they learned during Essentials that only including a couple cleric Domains (and expecting players to invent the other ones for the other gods) and only two of the four Seasons for the druids (and expecting the players to invent the Autumn and Winter), annoyed players to no end. ESPECIALLY considering they never got around to releasing more of them in later books.
Saving character generation details and options for "later books" only works for those classes that have an open-ended amount of details. But if you have a closed set (say the four Seasons of Druids or the nine Spell Schools), you'd BETTER include all of them together from the beginning. Cause if you only have the Sun and War Domains in that first Handbook and yet include an entire pantheon of gods to choose from... you're shooting yourself in the foot.