Okay, so I actually went down a rabbit hole on this once because I was trying to organize my Kindle library. Corinne Michaels has a lot of series, and most of her books are part of one. It's kind of her thing—interconnected standalones within a bigger world.
I'd say the vast majority of her novels are series-linked. The big ones are the 'Consolation' trilogy (that's three), the 'Salvation' series (which has... I think five? 'Beloved,' 'Beholden,' 'Betrayed,' 'Beloved'? Wait, no, I'm messing that up. 'Beloved,' 'Beholden,' 'Betrayed,' plus maybe a novella). Then there's the 'Returning' series ('Defenseless,' 'Relentless,' 'Reckless'), and the 'Hennington Brothers' duet. Even her more recent stuff like 'Say You'll Stay' seems to spin off into other characters' books.
So, to actually count them? If you mean novels proper, excluding short novellas, I'd ballpark around 15-18 of her books are part of a multi-book series. A few like 'The Regret Duet' ('Not My Match,' 'Not My Daughter') are technically two-parters, which she also counts as series. It's easier to think of which aren't—maybe a couple of her older, super early titles? But honestly, she builds entire universes.
My TBR pile is mostly her series now because you finish one and have to read the friend's brother's best friend's story.