There's no single, readily available statistic on the exact percentage of Americans who have earned college credit but not a degree. Data collection on this is complex, as it requires tracking individuals who may have attended multiple institutions and stopped at different points in their academic careers.
However, various studies and reports suggest that a significant portion of the population falls into this category. Estimates often range from 20% to 40%, but this is a broad range and can vary depending on the methodology and definition used (e.g., how much credit is considered "some" credit).