You can the boot following number of domains depending on your platform:
Up to 128 on UltraSPARC T2 Plus based servers
Up to 64 on UltraSPARC T2 based servers
If unallocated virtual CPUs are available, assign them to the service domain to help process the virtual I/O requests. Allocate 4 to 8 virtual CPUs to the service domain when creating more than 32 domains. In cases where maximum domain configurations have only a single CPU in the service domain, do not put unnecessary stress on the single CPU when configuring and using the domain. The virtual switch (vsw) services should be spread over all the network adapters available in the machine. For example, if booting 128 domains on a Sun SPARC Enterprise T5240 server, create 4 vsw services, each serving 32 virtual net (vnet) instances. Do not have more than 32 vnet instances per vsw service because having more than that tied to a single vsw could cause hard hangs in the service domain.
To run the maximum configurations, a machine needs the following amount of memory, depending on your platform, so that the guest domains contain an adequate amount of memory:
128 Gbytes of memory for UltraSPARC T2 Plus based servers
64 Gbytes of memory for UltraSPARC T2 based servers
Memory and swap space usage increases in a guest domain when the vsw services used by the domain provides services to many virtual networks (in multiple domains). This is due to the peer-to-peer links between all the vnet connected to the vsw. The service domain benefits from having extra memory. Four Gbytes is the recommended minimum when running more than 64 domains. Start domains in groups of 10 or less and wait for them to boot before starting the next batch. The same advice applies to installing operating systems on domains.