20 DBWR processes would never be active at the same time,.. On a 12 CPU
box, you still need to know the number of Disk access points, I.E. devices.
One question to answer would be how many devices I have to write to,.. maybe you have all your disk access thru a single volume (definitely not recommended).
I usually try to come up with a number of DBWR processes as a combination of CPU's available and devices that can be written to. 12 cpu's with 5 devices (logical or physical) I would have no more than 5 DBWR proceses.
It's all a matter of how many write processes can be active at the same time. Take into consideration the other processes as well, LGWR, etc. otherwise you will have processes waiting for CPU or the device to free up for another DBWR to begin.