These seemed to have been about 12 African American scientists that worked on the project at various points.

Mainly they were mostly chemists working within large teams such as Ralph Gardner who worked under Fermi on radioactive isotopes. Probably the most senior African American scientist was William Jacob Knox Jr. who was engaged in early attempts at gaseous diffusion (uranium enrichment) and was the only African American to become a project supervisor.

There were also a number of technicians on the project

Yes, there were many African-American scientists and technicians. Mostly they worked at the University of Chicago as part of the Metallurgical Laboratory, and at Columbia University. Not many at Los Alamos to my knowledge. I have written a bit about this here. They were, to be sure, underrepresented, as one would expect given the limitations on opportunity for African-Americans in the sciences in the 1940s.