What I think is most impressive is that he managed to write songs that are incredibly layered with dozens of interlocking parts, sophisticated chord changes, and complex rhythms and they were STILL major hits. "Sir Duke" was a #1 single. The album it is on had another #1 single and stayed at the top of the album charts for 7 weeks. And he wrote all these songs very quickly and at a young age.
By the age of 26, he'd released 18 albums. And between the years of 1971-1976, he released five albums widely regarded as masterpieces and won 12 Grammies including three albums of the year. Really as good a run of commercial, critical, and artistic success as anyone has had apart from the Beatles.