Is C3P0's knowledge of 6 million languages at or above average for a protocol droid serving in diplomacy? How many languages does the average protocol droid programmed for the job know?
It looks like 6 million languages is the base knowledge given to all 3PO droids by the TranLang III communications module, although the number of extra languages on top of this which are assimilated by a protocol droid in its lifetime will vary from individual to individual.
A TranLang III communications module allowed them to be fluent in over six million forms of communication, even if they could not respond in all of them for a lack of proper communication appendages. They also had the skills necessary to quickly analyze new unregistered languages and translate them into more well-known ones.
Sadly this is not cited to a specific source, but I did find a canonical confirmation here that it is the TranLang III communication module which gives C3PO his linguistic abilities:
The same information appears on SW Combine, but this is probably still not a canonical source:
Their TranLang III Communicator module allows the 3PO to comprehend over six million languages spoken by organic lifeforms, machines, computer systems and other droids.
The earlier model, the TranLang II communication module, was also used in certain protocol droids, and had a smaller base knowledge of only 1 million languages:
The TranLang II Communication module was the predecessor to the highly popular TranLang III communications module. The module stored around one million distinct forms of communication, and was the leading module of its day. It was still used during the Rebellion era, albeit not nearly as popular as the newer model. While not as powerful as the TranLang III, the module still found its way into protocol droids such as the J9 worker drone, and tutor droids such as the TTS-15.