I can't grasp how he did it. - Firms managing money on Bernie Madoff’s scale would typically have hundreds of people involved in administrative tasks.

From The New York Times:

[T]he 17th floor was Bernie Madoff’s sanctum, occupied by fewer than two dozen staff members and rarely visited by other employees.

And the 17th floor is now an occupied zone, as investigators and forensic auditors try to piece together what Mr. Madoff did with the billions entrusted to him by individuals, banks and hedge funds around the world.

[A] question still dominates the investigation: how one person could have pulled off such a far-reaching, long-running fraud, carrying out all the simple practical chores the scheme required, like producing monthly statements, annual tax statements, trade confirmations and bank transfers.

Firms managing money on Mr. Madoff’s scale would typically have hundreds of people involved in these administrative tasks.