Mike,
The rule doesn't limit how many rudders the boat carries onboard, it limits how many they can use overall.

I can see there being a limit on how many rudders they use overall because damaged rudders are slower than brand new rudders. Just like there is a limit on how many sails they can use overall. There is a strong resource allocation component to the race. They can't just build a new boat for each leg, even if they got the money and will to do so.