By 1990, the Census Bureau included more than a dozen ethnic/racial categories on the census, reflecting not only changing social ideas about ethnicity, but the wide variety of immigrants who had come to reside in the United States due to changing historical forces and new immigration laws in the 1960s. With a changing society, more citizens have begun to press for acknowledging multiracial ancestry. The Census Bureau changed its data collection by allowing people to self-identify as more than one ethnicity. Some ethnic groups are concerned about the potential political and economic effects, as federal assistance to historically underserved groups has depended on Census data. According to the Census Bureau, as of 2002, over 75% of all African Americans had multiracial ancestries.
The Census Bureau had gone from two categories to how many by the 1990s?
more than a dozen
According to the given context, several ethnic groups were included in census by 1990. In fact it is mentioned that as of 2002, over 75% African Americans had multiracial ancestries and this had changed from two categories previously.
Thus, more than a dozen ethnic/racial categories must have been there on census by 1990s.