Question is why were there so many? Was it just a writing gaff?
No, if wasn't a gaff. They intentionally planned to have rotating engineers in the first season. I believe it was one way Roddenberry wanted to distinguish the new series from the old. I've head this discussed in several documentaries, but what I've not heard discussed in any great detail is what made them changed their minds in season 2 and made Geordi permanent chief. I can speculate and make educated guesses, but it would be nice to have the rationale confirmed by someone in the know.
).