Will there be a collision detection in PVP? Or can i can just run through enemys?

In my opinion its necessary for action based combat system! But is it even possible with hundreds of player on a map ?

Good question, at first I thought you was asking about destructive environments like destroying siege walls (which would be a yes).
Anyway, I'm pretty sure there's collision detection, watching the gameplay videos you just get a feel like there is, and if they're staying true to traditional ES combat you won't be able to run through people.