Hi there,
There does not seem to be a limit on the number of cores/threads for Swarm, I have seen a few posts where they were using 72 or higher number of threads. Epic uses a network of servers that could potentially have 5-10x (possible more) more than a dual 18 core Xeon setup.
Here is a quote from the release notes of UE3 2009-11 Beta explaining how it works:
Unreal Swarm is a massively scalable job distribution system optimized for high-speed networks of multi-core PCs. Designed in tandem with Unreal Lightmass to accelerate lighting computations, Swarm can manage clusters of computers of mixed brands, configurations, and performance levels, automatically distributing tasks on-demand and employing a transparent distributed cache system that allows for dramatically increased performance.
The only documentation currently available for swarm can be found here:
http://udn.epicgames.com/Three/Swarm.html
So whether you are building a server farm, or a single high powered server, you will be able to make use of all those cores. 