Keillor said that Dune grew out of Herbert's work on ecology, and was rejected by 23 publishers; Wikipedia says 20. Either way, a lot of editors were kicking themselves later. Herbert eventually got the novel published by Chilton's, the folks who publish car repair manuals.
So, yet another example of persistence paying off. If Herbert had given up after publisher 19, who would have heard of him—or Dune—today?
To quote Keillor's signature sign-off: "Be well, do good work, and keep in touch®"