There's no recommended ratio as such beyond "as long as your server copes, it's fine".
There's no single common use-profile for those ... and if you've got a replicator hanging off a 33 Kbps modem (for argument's sake), even 1 preferred server may be too much, as it were .
Similarly, you can "configure yourself" in (or out of) trouble. By having different Preferred servers hit up the replicator(s) at different times, you balance the load -- assuming you have that luxury - for instance. So ... there's a lot of "it depends" type stuff here.
As long as your replicator is able to handle the load, it's fine. If it can't handle the load (because you MUST get X data across in Y amount of time window, for instance), then you need to look at increasing the replicator count.
Not the most helpful of answers, but I hope it explains why this is a "how long is a piece of string" type question? .