Various researchers have repeatedly studied the number of bugs per 1000 lines of code (e.g., Basilli and Perricone, 1984; and Ostrand and Weyuker, 2002)

Various researchers have repeatedly studied the number of bugs per 1000 lines of code (e.g., Basilli and Perricone, 1984; and Ostrand and Weyuker, 2002). Bug density depends on module size, module age, and more, but a ballpark figure for serious industrial systems is between two and ten bugs per thousand lines of code. This means that a monolithic operating system of five million lines of code is like- ly to contain between 10,000 and 50,000 kernel bugs. Not all of these are fatal, of course, since some bugs may be things like a minor misspelling in an error mes- sage is rarely needed.

Andrew S. Tanenbaum (Author), Modern Operating Systems