Typically we work on commodity server hardware. So intel has plans for a 6 or 8 way chip and then you're likely to see 4 socket version of this. So 24 or 32 processors. There have also been some special 8 socket opteron motherboards so you could conceivably have 8 quads for 32 processors but these are very rare and very expensive. The 4x4 machines are hard enough to come by.

The other option for more cores is sun's T2 chips which have 64 threads on them. This is a very different beast from a real 64 processor machine but it might be useful for working on general kernel scalability. We do have a port for these platforms but it's not well maintained.

In the end it comes down to what we can get our hands on. I can't afford to purchase every new generation of parts. Although I will probably try to get something with 32cores once that is mainstream as an upgrade over my current 8way systems.

Linux has the advantage of being backed by some companies with very large platforms of less tightly connected (NUMA) nodes with as many as 1024 cpus. We just don't have this resource and that isn't our core focus anyway.