They do not give numbers, but I also doubt that "largest" claim.
The USA is a lot larger, but the Netherlands has a national register of every bicycle reported stolen to the police (https://fdr.rdw.nl; very much spartan interface). They do not give numbers, either, but at 100k+ reported bicycle thefts a year and running since 2008, there must be over half a million stolen bicycles in their database. Google tells me that newly sold bikes also end up in that database. I don't see how they would need that data, but the site reportedly _started_ with over 4 million bicycle numbers.
The USA is a lot larger, but the Netherlands has a national register of every bicycle reported stolen to the police (https://fdr.rdw.nl; very much spartan interface). They do not give numbers, either, but at 100k+ reported bicycle thefts a year and running since 2008, there must be over half a million stolen bicycles in their database. Google tells me that newly sold bikes also end up in that database. I don't see how they would need that data, but the site reportedly _started_ with over 4 million bicycle numbers.