Here's the question and answer from my Bank Note Clinic column:

Q: I've read about "packs" and "bricks" of bank notes. What are they talking
about?
The Bureau of Engraving and Printing prints all U.S. notes. A pack of notes
contains 100 notes. The packs are made into a brick of 40 packs or 4,000 notes
for shipment to the Federal Reserve Banks. You can ask you bank for a pack, as
this would be the form in which they get them. The notes in the pack would
normally have serial numbers in sequence, with replacement notes - which
have a star instead of a letter in the serial number - filling in for any
defective notes caught during the printing process.
Alan Herbert
The AnswerMan