Issac Asimov wrote nearly 500 full-length books in his life.
He has also written over 90,000 letters while keeping a monthly column in the Magazine of Fantasy and Science Fiction for 33 years between 1958 and 1991. He also kept his job as professor of biochemistry at Boston University, during which he completed 28 novels and a list of short stories.
Here is interesting story on what made him leave his job of professor at Boston University.
With him having so much success with writing, he was forced to choose between : Be an academic or be a popular writer.
He writes in his autobiography :
I finally felt angry enough to say, “…as a science writer, I am extraordinary. I plan to be the best science writer in the world and I will shed luster on the medical school [at BU]. As a researcher, I am simply mediocre and…if there’s one thing this school does not need, it is one more merely mediocre researcher.”
[One faculty member complimented him on his bravery in fighting for academic freedom.] I shrugged, “There’s no bravery about it. I have academic freedom and I can give it to you in two words:
“What’s that?” He said.
“Outside income,” I said.
That is good guidance for anyone looking for freedom.