I can never keep this stuff straight. Looking at my bookshelf right now, there are four Atlee Pine novels before the latest one. The first is 'Long Road to Mercy', then 'A Minute to Midnight', 'Daylight', and 'The Sixth Wickedness'. The new one is 'Mercy'. So if you're asking about the number you need to read before the current release, that's four.
I'd honestly suggest reading them in order if you can. Baldacci builds Atlee's backstory about her missing sister across the early books, and while each has a self-contained thriller plot, those personal threads accumulate. Skipping 'Long Road to Mercy' means missing the foundation of her character and her drive.
That said, I tried 'Daylight' without reading the first two and was a bit lost with the references to prior cases and her FBI partner, Carol Blum. It's doable, but you'll get a richer experience from the sequence. The fourth book, 'The Sixth Wickedness', feels like it wraps up a major phase of her arc, so starting with 'Mercy' after that might be a decent jump-on point if you're pressed for time.