Thats highly dependent on the workload.
If all 15 are mostly idle, then a single dual core CPU might suffice.
If all 15 are very active, you'll need a pretty beefy box.
If they are, as you say, pretty active, you should be looking at providing at least 12-16 cores to the system, and ~32GB of memory. Physical design doesn't matter as long as it can fit the resources needed.