Hi,
there is no recommended ratio. The number of publish instances should only be driven by the load you expect at publish level. A typical number of publish instances is 2 to 6. A higher number of instances can cause performance problems on authoring side. If you plan to have more than 10 publishs, I would strongly recommend to get your architecture reviewed by Adobe consultants.
The number of authors is mostly determined by the type of application; and clustering won't help to scale the replication, since the replication runs only on the cluster leader.
kind regards,
Jörg