Well the dj would presumably be searching within the theoretical space, since he wouldn't have access to the list of actual serial numbers printed.
Does anyone know if the serial numbers are printed in order? I didn't see the answer in a very cursory web search. If they are, presumably it would be trivial for such a contest to avoid ever paying out -- just choose from the upper end of the SN range.
Afaik, the only time serial numbers get reused is when a bill is reissued because it is damaged or worn, in which case they would shred the old bill (also, the new bill would get a neat star next to its serial number).
Depending on which parts of the serial number the DJ read off (just number, or number and letter), there could be up to 832 bills for a single serial number: http://onedollarbill.org/decoding.html
That's generally what the "serial" part of "serial number" means. And why the smarter bank robbers ask for non-sequential bills, because new bills generally have consecutive serials which are simpler to trace
1 Trillion is 150 singles for every human alive