Oh, that's a slightly tricky one because 'best-selling series' could mean a couple things. She's known for a few long-running ones that all seem to sell well. My gut says you're probably asking about her 'Rylee Adamson' series, which is the one that really put her on the map for a lot of urban fantasy readers. If that's the case, the main story arc is a tight ten books. It feels complete, too, which I appreciate. I got so attached to those characters, and the way she wrapped it up felt right, not dragged out.
But I'd be careful just counting that. She also has a spinoff series called 'Tracking' that follows one of the secondary characters, so some folks might consider those part of the overall 'Rylee-verse'. And her 'Elemental' series and 'Desert Cursed' series are also super popular in their own right. So the exact number depends on where you draw the series line.