For single cores to get maximum turbo clock the other cores have to be at least in C3 ("The core flushes the contents of its L1 instruction cache, L1 data cache, and L2 cache to the shared L3 cache, while maintaining its architectural state. All core clocks are stopped at this point. No snoops."). Of course you can set some BIOS/EFI so that it doesn't care about the number of cores, the results then depend on your CPU.
My 4790K at 1.271v doesn't run stable at higher than 4.4 gHz with all cores being active. It does run at 4.5 to 4.6 gHz with one or two cores, though. And since most software only uses 1-2 cores regularly it makes sense to set my BIOS in such a way that 1-2 cores get higher clocks.
This is the default behavior, too. The idea behind that behavior is that less cores draw less power and create less heat, thus they can be clocked higher without issues while the whole CPU package is getting cooled (heat spreads to offline parts of the CPU as well).