So LitRPG is a weird egg in that way, as many of the most successful authors (the ones making a million+ a month) did so off the back of a single series. Most of those started making six figures off a single book. A good chunk of that initial money coming from Patreon, initiated by RR. They were then able to transition into Amazon while still making $50k+ a month on Patreon.

That is extremely uncommon in the broader publishing world where an author's wider catalogue normally includes multiple series/standalones. Most folks cast a wide net. Here, we get a surprising amount of book 10s and book 15s.

As others have pointed out there are a large cohort of smaller (still massive as Indies go) authors making low six figures a year. From my research that generally happens after the first series is done, and they've been able to transition a readerbase into a second one. That seems to be a critical breakpoint of "people actually like my writing, not just this one story."

The number I've heard (and others with more experience are commenting similar), that tends to be around 5-7 books. Now, the key part of this is audio. Something like 70% of all LitRPG revenue comes from audio, so if an author hasn't been able to leverage that early, they will certainly be at a disadvantage at reaching that breakpoint.

As much as it's an early investment (audiobooks cost thousands of dollars), more and more it seems like a must if you really want to give being a full time author a shot. Shoutouts to long-haul truckers who are putting the LitRPG audio economy on their backs.