Quoting the UltraLinux web page (only SMP systems):
UltraLinux has ran on a 14 CPUs machine (see the dmesg output).
(David Miller) There should not be any worries.
The only known problem, and one we don't intend to fix, is that if you build an SMP kernel for 32-bit (ie. non-ultrasparc) systems, this kernel will not work on sun4c systems.
(David Miller) There is a bug in the include/linux/tasks.h header file, it needs to define NR_CPUS to 64 on UltraSparc as this is the upper limit for the hardware we support :-)