Someone else already posted an explanation, but it just boils down to us Danes still using “snes” (or “snaes” in old english), which was the amount of dried herrings usually hung from a wooden drying rig/branch, which, of course, was 20 (edit: according to this source).

It’s totally disconnected from modern times and ought to be changed, but then again, some people still use the 12-hour clock, thumbs, and feet, so how bad is it really?