Do writers earn a high income? How many are there in general? What's the difference between online novels and publishing physical books?

Whether it is high or not depends on how you write.

Nowadays, online writers earn millions of dollars a year, but those who struggle to get the minimum living allowance every month depend on how they write.

The biggest difference between online novels and physical books is that as long as you publish online novels, people will read them, and you can keep a certain amount of manuscripts. Moreover, some authors don't have manuscripts, and substantive novels can only be published if they are written completely, and some people may not value them when they are finished.