I've written a few not-very-good books in the fantasy realm. One struggle I've had is juggling multiple plotlines. For the first book I wrote I wanted to write a twisty, intricate story with a lot of characters whose backstories all intertwined. I think it failed because in the end it was too convoluted and even I couldn't keep all of it straight. For the next one, I focused on a very simple plotline and one love interest side-plot and it felt paper-thin and predictable. I'm wondering if there is a rule of thumb for how many different stories can intertwine in an average-length novel.
Any research I've attempted to conduct mentions lots about story arcs and character development, but nothing seems to answer this question directly.