I am considering putting a limit on the maximum doom I can have as a GM. The reason is when my players get desperate and start spending doom like crazy my pool gets over saturated to the point I can’t spend it all.
I am thinking somewhere between 5 to 10 doom per player. Heroes can only push their luck so far before they have to accept fate.
Or do like our GM does. 
Start spending them to complicate things, I promise you they will be far more careful adding Doom to you.
Some good ideas for Doom:
Reinforcement arrives for the enemies
a minion or two bedomes a toughened
A fire starts and smoke makes visibility hard
etc etc
I concur with Mogge.
Use Doom for what its there for, complications, environmental changes, hazards, reinforcements, additional dice for your NPCs, sorcery. Use it actively, the more the players give you doom, the more you should spend doom - although saving some for the climactic encounter of the session is always a good idea.
Having a limit on the doom pool is bad idea, as this introduces another meta-play from your players. Better devise new ways to spend it, rather than giving them a tool to use doom without further consequences.
The pool resets each session anyway.
I would advise against that. Use Mogge’s advice. Use the doom to make things interesting…be very creative with what their desperation has wrought.
Dont forget extra dmg, and remember also: VIP NPCs can spend 3 Doom to use a Fortune point!
And also use Doom to let the NPC’s start before the PC’s every now and then to spice things up.
Don’t limit yourself. That gives the players an easier out. If you use the doom as it comes in you won’t find it building up so much. As others have said, try spending it to create complications or other interesting things. There’s a lot you can do and your players should be just as nervous about giving you the doom as they are failing.
There are a bunch of threads with ideas for creative doom use. Just delve a lil deeper in Tsontha-lanti’s dungeon maze!
Those should be consolidated into a thread, that is pinned to the top of the Conan sub-forum. If anyone has the time and inclination to do the job … 
Personally, I find that spending Doom regularly helps avoid this most of the time - spending 1-3 points per round in combat, and a few points routinely on small things can help keep it building up. Only let it build when you specifically want a large quantity for a later scene, and then spend at a higher rate once you reach the point when you want to spend again. It’s a dramatic pacing tool, and the rise and fall is something the GM should control.