Zip, zero, zilch. If you are eating a well balanced diet, then you should not need so many vitamins and supplements. I do take echinaccea if I am sick, but that's it.
I have heard doctors say this for years. But I think it reflects a mindset that vitamins are just food. If you see vitamins and herbs as medicine, then you can see that there isn't any way you could get it all from your diet. You don't regularly eat echinacea flowers as part of your diet, right?
Think about fish oil. Studies have shown that fish oil in high doses can dramatically improve focus and behavior in children with ADHD. You can't get enough fish oil in your diet to effect that level of improvement, because it isn't safe to eat that much fish anymore, due to mercury. You could put your child on a pharmaceutical to improve focus; no one would say you should be able to get that effect from a balanced diet. If you are using fish oil in the same way, as medicine, you have to take it in doses that you can't get from a balanced diet. Not everyone needs that level of fish oil in their diet.
I take B6 every day because I have a B6 deficiency. My asthma medication causes an inability to properly use B6 in some people. I cannot get enough B6 in my diet to make up for what the medicine keeps me from being able to use.