I don’t know exactly how many supplement brands there are, but I’d be willing to bet there are hundreds of brands that do between $5 and $100 million dollars per year in revenue. Many of these brands exist almost entirely because they’ve figured out how to sell on Amazon, but some have even ended up in other famous supplement retailers.
Amazon doesn’t vet these companies. So far they’ve been shielded from lawsuits because they hide behind being a marketplace and not a retailer. This is a legal loophole but it is the current situation. That means Amazon doesn’t really care if the supplement companies are selling products that are mislabeled or filled with inert non-listed ingredients, etc. It doesn’t care if the ingredients are actually organic, etc.
Another thing to consider is how are so many small companies able to offer such a wide range of products? Are they really able to do this? The answer is no. Most supplement companies buy from the same few manufacturers. Most of those manufacturers are based out of China.
So should you order supplements? I have no idea. Probably just eating as few junk foods as possible and eating as many real foods as possible is the best thing you could do. If it makes you feel better and you want to take supplements I would advise using a major retailer because they have processes in place to ensure compliance with laws.