The United States has one central bank: the Federal Reserve System ("the Fed").
Created by the Federal Reserve Act of 1913 after a series of financial panics, the Fed is a unique public-private hybrid:
Plus: Moderate long-term interest rates, financial stability, consumer protection, payment system efficiency.
The Fed is independent within government: decisions don't require presidential/executive approval; funded by its own earnings (not Congress appropriations). But it's accountable: Chair testifies to Congress semi-annually; publishes minutes, transcripts, reports; subject to GAO audit (except monetary policy deliberations).
While there are 12 Reserve Banks, they are branches of a single central banking system—not separate central banks. The Eurozone has a similar structure (ECB + national central banks), but the Fed is a unified entity.