L. A. Times:

Sproul said his company has a vigorous quality control system that includes running criminal background checks on all employees, requiring viewing instructional videos on registration laws (an example of which can be seen here) and cataloging voter cards with serial numbers that identify who collected each registration. That quality check, Sproul said, enabled the company to quickly determine the individual who submitted the problematic cards in Palm Beach County.
…..
The company helped identify 106 forms submitted by the same worker. Bucher turned them over to Palm Beach County prosecutors, who have begun a criminal investigation.

Chicago Tribune:

The Palm Beach County elections office first reported finding 106 potentially fraudulent registration forms earlier this week that had been submitted by Strategic Allied Consulting (SAC), a Virginia firm hired by Florida’s Republican Party.

Since then scores more suspicious forms have been detected in at least five other Florida counties where election officials say SAC worked to register voters.
articles.chicagotribune.com

So, maybe not the work of just one guy.