So far we know, or can strongly assume, that the following Pokemon are in the Kalos Pokedex:
EDIT: I've added the latest rumoured Pokemon (Pangoro and the 4 after Gogoat) to the list, simply because the source has been reliable in the past.

#001 Chespin
#002 (Chespin evo)
#003 (Chespin second evo)
#004 Fennekin
#005 (Fennekin evo)
#006 (Fennekin second evo)
#007 Froakie
#008 (Froakie evo)
#009 (Froakie second evo)

#--- Fletchling
#--- (Fletchling evo)
#--- (Fletchling second evo?)
#--- Helioptile
#--- (Helioptile evo?)
#--- Pancham
#--- Pangoro
#--- (Gogoat prevo) [link]
#--- Gogoat
#--- Inkay
#--- Malimar
#--- Espurr
#--- Meowstick

#--- Pichu
#--- Pikachu [link]
#--- Raichu

#--- Azurill
#--- Marill [link]
#--- Azumarill

#--- Eevee
#--- Vaporeon
#--- Flareon
#--- Jolteon
#--- Espeon
#--- Umbreon
#--- Glaceon
#--- Leafeon
#--- Sylveon

#--- Xerneas
#--- Yvetal
#--- Mewtwo

Which section of the Kalos Pokedex (Central, Coastal or Mountain) can be debated, but most listed here are likely from the Central with the exceptions of the Marill-family (Coastal), and maybe Helioptile (Mountain)


And IF Fairy-Type is a new Gen 6 Type, then we could assume (heavy speculation) that the following could be Fairy type, and thus maybe feature in the Kalos Pokedex:

Cleffa - Fairy
Clefairy - Fairy
Clefable - Fairy
Igglybuff - Fairy
Jigglypuff - Fairy
Wigglytuff - Fairy
Happiny - Fairy
Chansey - Fairy
Blissey - Fairy
Togepi - Fairy
Togetic - Fairy/Flying
Togekiss - Fairy/Flying
Sylveon - Fairy

Snubbull - Normal/Fairy
Granbull - Normal/Fairy
Azurill - Water/Fairy
Marill - Water/Fairy
Azumarill - Water/Fairy
Mawile - Steel/Fairy
Minun - Electric/Fairy
Plusle - Electric/Fairy
Pachirisu - Electric/Fairy
Audino - Normal/Fairy
Cottonee - Grass/Fairy
Whimsicott - Grass/Fairy


So, from speculation and assumptions, this means that there are 61 (36 from artwork and assuming evolutions and families, 4 from rumoured new Kalos 'Mon, and 21 further if the Fairy speculations have anything to go on) possible Pokemon listed here for Kalos so far, and it would be a safe assumption that the Kalos Pokedex will include approximately between 250 and 350 Pokemon (I'm betting between 300 and 350), similarly to the Unova Expansion for B2W2, since we can already see from Artwork that some older Pokemon will feature, and we can assume more will feature as well.