This is a terrible question. It doesn’t really mean anything other than “on average, we see XYZ wordcounts in this genre.” And it says nothing about why.
Here’s a better question:
When do readers of this genre want to see subplots, and what kind?
A novel with one main plot is about 40-50K. For example, The Stepford Wives is about 45K. Most pulp novels have one main plot. The early Doc Savage books are about 50K each.
Epic fantasies, contrariwise, have lots of subplots. In fact, the readers would be upset if there weren’t subplots. The Wheel of Time books are about 300K each.
In between are the things like The Maltese Falcon, which has the main plot (falcon/murder) and a subplot (love triangle). 67K.
Subplot = wordcount.
“Let’s talk about the current political situation at length” is a supblot. See 1984 by George Orwell (88K), which has a romance, a rebellion, and a political treatise.
I could go on, but let’s say in general, a single subplot is about 15-20K of sheer words (about a novelette’s worth of plot*). And that anything over 50K should have subplots.
And that when you get an arbitrary “how long your book should be” number from an agent or editor (or even just checking word counts on bestsellers), what you’re seeing, in general, is how many subplots are popular in a novel these days.
*I’ll post my best guess as to GRRM’s structure on a different day. It’s different than this.
I’m trying to find an “ask” format that I feel comfy with. I still haven’t yet. I’m going to keep posting this one for a while, at least until I come up with something better. Better than nothing, this one is, but not by much!