If all other specs stay the same, you probably cause an CPU to memory imbalance. A quad core being used for VMs might run 4. A 12 core could run many more, but the memory would need to be tripled to scale appropriately.
As was said, otherwise it will be dependent on the software being used. Some software will scale fairly linearly up to 4 or 8 cores. A very small percentage of software has been designed to scale beyond 8 cores. Even if the software scales beyond 8 cores, it is difficult to keep 12 cores fed with data. You need high bandwidth storage or network. It is very easy to have an unbalanced system that has idle cores.