There was a period of about a month where I only read one book, but all that meant is I was reading War and Peace. Likewise I can read multiple books a day for a few days straight, and all that means is I'm reading some light novel/manga series. And of course VNs aren't even technically books, but I still count things like Fate/Stay Night or House in Fata Morgana as reading.

My goal is just that I spend a reasonable amount of my free time reading. Obviously that's impossible to quantify, especially since it depends on how much free time I have. But its the only reasonable metric I've found, even if I just have to go by feel.