They have announced 4 open world maps, some with 3 biomes (Roots, Floor and Canopy).
You'll journey through four of the largest, most diverse and replayable maps ever built in the world of Guild Wars®; maps in Guild Wars 2: Heart of Thorns are designed as three-dimensional volumes rather than traditional flat planes. You'll have more to explore as you travel through the jungle biomes, from the canopy heights to the jungle floor and deep below into the jungle roots.
http://heartofthorns.guildwars2.com/game/maguuma
These are the map names that have been datamined:
Verdant Brink
Ebullient Depths
Auric Basin
Dragon's Stand
This of course does not count the Guild Hall maps or Borderlands or Stronghold maps or anything else.