I'd guess that would be few. But what amounts to an overdose seems to vary by individuals and, from what I've read, some people do overdose. By 'overdose' I mean an amount where the effects become deleterious.
You can check the poison center. That is where it would fall under. I already know the answer to both.
1) Zero
2) Over 100,000 annually.
This is real simple. Follow the money. There is BIG money in pharmaceuticals compared to the Vitamin industry. No comparison what so ever. Look how much the average multi vitamin, or even Vitamin D. Compare that to the average cost of a pharmaceutical drug.