Complex sentence: From 1916 through his Kennedy Memorial cover on December 161963, Rockwell created 321 magazine covers for the "Saturday Evening Post", which was the most popular American magazine of the first half of the 20th century.
Split into simpler sentences: From 1916 through his Kennedy Memorial cover on December 161963, Rockwell created 321 magazine covers for the "Saturday Evening Post". The "Saturday Evening Post" was the most popular American magazine of the first half of the 20th century.