Recently I did a series on the Star Weekly Novels which were published from 1938 to 1973 as inserts in the Saturday Toronto Star, one of 21 US (18) and Canadian (3) newspapers with novel inserts that I've so far identified.
There is very little information on these inserts on-line but I've cobbled together lists totaling 2500. An early guess is at least 12,000 were published. The dates so far are 1910/11 and 1928 to 1973. At least six US newspapers used inserts produced in Canada, just changing the newspaper name on the first page.
Here are three US examples. The Philadelphia Inquirer Gold Seal Novel was published at least from 1934 to 1949 and, unlike the Long Island Sunday Press, were original novel inserts.
The Philadelphia Inquirer, Sunday, November 2, 1941 - The Affair of the Circus Queen by Clifford Knight, illustrated by Harry Weinert
The Philadelphia Inquirer, Sunday, August 10, 1941 - A World in Spell by D. E. Stevenson, illustrated by Harry Weinert
Long Island Sunday Press, Sunday, November 13, 1949 - Lord of Atlantis by John Russell Fearn, illustrated by William Book. First published as Toronto Star Weekly Complete Novel, October 8, 1949