There is a lot of work required and time needs to be spent on upkeep. My courses take around 6-9months to create and they need to be well planned and they require a lot of preperation. Various areas are very saturated and so if you don't bring your "A game" as Udemy likes to put it, then you are unlikely to get sales. Course also need to be updated (especially in the technology sector, as things move fast).
I also spend a good couple of hours a day just answering questions.
My suggestion is to take being an instructor seriously. Students want good content which needs to be well planned and easy to follow. Students also want fast responses to their questions. There are a number of courses on Udemy that have been quickly put together or badly planned and most of thise courses aren't getting enrollments or generating sales. The reviews also reflect how students feel.
In short, don't underestimate the work required. Things take time and it requires dedication. You can make money but realistly I wouldn't call in "passive income". You get what you put in.