Am I correct in assuming that there is no limit on how many Doom Points I can spend in a round?
So for example can I use - let´s say - 9 Doom Points from the Pool after a hit to add 9 points of damage and maybe bring the PC to 0 Vigor with that hit?
You can do this, but why would you do it?
Just to cause a Wound on a PC or even 2 in that case (5 or more damage in one hit AND Stress track falls to zero = 2 Wounds)?
Damage does not make things more interesting in itself.
The Doom pool is to make things more challenging and more interesting and thus more entertaining to the players.
Do you really think just adding a vast amount of Doom to force 2 Wounds on a PC will make that player come back for another session of Conan?
There are some, somewhat high, Doom spends I recommend: using Doom for NPC sorcerers to activate their spell’s Momentum spends. That way an NPC sorcerer can be as dangerous and frightening as they should be.