The percentage of adults in the United States with a college degree varies depending on the source and how "college degree" is defined (bachelor's degree only, or including associate's degrees). However, a generally accepted figure is that
around 35-40% of adults in the US have at least a bachelor's degree. This number fluctuates slightly year to year.
To get the most up-to-date and precise figure, you should consult the latest data from the U.S. Census Bureau or the National Center for Education Statistics (NCES).