>Does that mean registered or that showed up at the event?<
In my case, it was the number of boats registered. This is another can of worms, for discussion, and needs to be clarified for those who go by the strict letter of the rules. You would think common sense would dictate the number who showed up at the Regatta.

It is a tiny difference with a few boats, but when you are looking for one point it's bigger.
In a large fleet the point difference could be huge, some may just go to the start/finnish line then retire to save on points.
If all things were equal in number of boats, why the difference between a series vs a regatta?
A three race regatta vs a 12 race regatta. How many races does it take to justify a series scoring method (13?). Or a series could be 3 races over 3 weekends.
I do not see why it is necessary to score it differently.
Regardless when did this change?