The US is huge. 278 whistleblowers from 2012 to present is not bad at all.
1. Had the whistle blown
2. Were investigated
3. Had enough evidence found during the investigation
4. Were fined successfully
5. Were announced via the SEC whistleblower program
Nobody knows the dropoff at every stage but it's certainly non-zero and so we can't judge the level of crime based on the number we know coming out at stage 5.
I don't think it's overly cynical to assume that 278 is a small percentage of the actual count of financial crimes in the US since 2012.
We'll never know the exact details, but we can still piece together a statistical model of the world from the evidence we do have:
We can compare it to similar systems, for example. Murderers have to go through similar steps, so one might reasonably conclude that high-value financial crimes are less common than murder. If there are other countries with similar programs, we could also compare our number to theirs.
Plus, why would that comparison be useful? Everyone is capable of murder, not everyone is even in a position where they could attempt to commit the types of crimes that get announced via the whistleblower program. So if we were to compare, it would have to be some kind of per capita comparison, where the population for financial crimes is members of the financial industry. In which case the numbers start to look more damning.