What's the maximum seat configuration for a 747 that is actually used?
The maximum was likely 568 seats.
It is hard to find a definitive source for this, but it is likely that the 747-400D (Domestic) version had the largest number of seats since it was specifically built for this purpose. This type was used by Japan Airlines (JAL) and All Nippon Airways (ANA). JAL published the number of seats they used in a press release:


How many seats in JAL's passenger 747s: We have many configurations for international and domestic airplanes. The latest international route 747-400 has 348 seats - 12 in First Class, 99 in business class and 237 in economy class.
Our domestic Short Range 747s have as many as 568 seats in two classes: 24 seats in the domestic version of First Class, which we call Super Seat, and 544 seats in regular economy class.



(jal.com)
Here is the seat configuration used by JAL:

(image source: simpleflying.com)
I did not find a statement by ANA itself on their seating. The following article however lists 565 seats as their configuration, so 3 less than JAL:

The Boeing 747 – once dominant in the sky is slowly fading away. The latest airline to wave goodbye to the Boeing  747 is All Nippon Airways, who completed the final flights on its domestic routes.
It marks the end of Boeing 747-400s by a Japanese airline – and also the end of an era of the passenger heavy Boeing 747-400D aircraft.
These aircraft had a massive passenger load of 565 seats on a plane.

(economyclassandbeyond.boardingarea.com)
While not impossible, it is unlikely that another airline would have ever operated a 747 with a higher seating capacity.