From the FactTank story:
The new category would be broader than the Arab ancestry data collected by the Census Bureau since 1980. The Arab-American population is small but growing, and its exact size is disputed. The Census Bureau estimates there are 1.8 million Arab-Americans in the U.S., up 51% since 2000. But the Arab American Institute Foundation estimates there are nearly 3.7 million Arab Americans living in the country. The Arab-American population is also diverse, with people claiming ties to 22 countries and various religious backgrounds.