No, zergs will be limited due to three important factors:
1) player-to-player body collision: you cannot walk through a player, and you can block another player with your avatar's body; you can also rush and push someone out of the way which is a combat tactic
2) the immense size of the world: local communities form that have land ownership, along with racial warfare, limit certain factors; but more importantly, it takes time to get from one place to another because the world is so large... no instant transportation, it takes planning and organization
3) very strict limitations on teleportation abilities: runes are required that are a) expensive, b) take a long time to cast, c) lootable by anyone & rare, d) must be attuned to the destination beforehand at the destination (attuning taking up to 10 minutes without interruptions), and e) can only teleport one person (the user)
There will be no zergs, really. There will be large battles though. Which will be a blast.