Man, I've been through this series a couple times and I've always seen it listed as a trilogy. 'Cold Magic', 'Cold Fire', and 'Cold Steel' are the three main novels that complete Cat Barahal's core journey from an unwanted marriage to a world-changing revolutionary. There's also the short story collection 'Cold Magic: The Spiritwalker Trilogy,' but that's more of a bind-up or a companion piece. I'd say it's firmly three books.
Sometimes you'll see 'Cold Steel' split into two volumes in some international editions, which can cause confusion, but for the primary English-language releases, three is the number. It's a complete arc, and honestly, Elliott wrapped it up well—I'm glad she didn't stretch it out.