I couldn't find anything either - so I just copied the value that the VM was getting launched with. On my 15" Core i5 MBP, as well as my 2009 13" MBP - they were getting launched with 2 cores.

Maybe VirtualBox has some logic to check for multiprocessor if you don't specify the number of CPUs - and if it finds a multiprocessor system it just sets it to two cores (instead of maxing it out). I think maybe I should just not specify a default value.