I've rarely seen a penalty for having too many cleaners configured. The exception would be when you have many chunks on few slower spinning disks (as opposed to SSD), in which case, I wouldn't go more than 2 per disk. Problem is, with modern storage and SANs, you may not actually know how many disks there are behind the scenes. In that case, I'd generally follow Art's recommendation of one per frequently-written chunk.
Like Tom, I have rarely seen any issues with too many CLEANERS. Unless it is a small system I tend to set at 128 and leave it alone