This particular error message (ERROR ) in many cases is either a "bogus" message or just the consequence of the "real" error. It can happen, for example, if a Java transformation doesn't get any input. It did happen in some older PowerCenter version if a Sorter didn't get any input.
There are a few other cases as well, but in order to clarify this we would need to see both the session log and the actual mapping.
Hence I am 100% sure that this has nothing to do with the count of replacement names in the dictionary files.