There is no authoritative list of government agencies. The Federal Register indicates there are over 430 departments, agencies, and sub-agencies in the federal government. However, there are other lists. For example, FOIA.gov lists 78 independent executive agencies and 174 components of the executive departments as units that comply with the Freedom of Information Act requirements imposed on every federal agency. But this appears to be on the conservative end of the range of possible agency definitions.

Source: Forbes