There's no definitive, readily available count of how many one-room schoolhouses remain in the United States. The number fluctuated greatly over time and many have been demolished or repurposed. Furthermore, a comprehensive historical record tracking every single one-room schoolhouse across the entire country was never systematically kept.
While some states and organizations have made efforts to document surviving structures, it's a piecemeal process and wouldn't represent a complete picture of the total number that *ever* existed. The peak number would have been in the late 19th and early 20th centuries, but even that figure is impossible to precisely determine.