I confirm that on my '96 E300, #21 is the engine RPM in hundreds. It shows 6 at idle, with the instrument panel tach showing 650-700.

However, I don't think that the N22 has a separate tach input. I'm pretty sure that the instrument panel and the N22 tachs are fed by the same sensors, but I don't recall which feeds which. Perhaps the signal goes to the N22 over the serial interface on my car, along with the ECT (#6). Yours is newer so it's probably a bit different, but I still doubt the presence of multiple sensors.