More people means more consumption and waste. You might argue that two careful people could consume less than one average person which is true, but statistics don't bear this out. Thus, on average two people consume more than one. 4. It is not known, but I believe is is in the tens of thousands from what I have heard about the destruction of the rain forests. But the rain forest is only one of many environments destroyed.
People require space which takes away space for other species.
It's a zero sum game. Well this is a good questions, but I don't think we'll know the full answer to it for years. We do know that American's have a serious weight problem, so something is wrong. I suspect that all of the modifications people have done to food to make it sweeter, etc. has upset the nutrient balance within natural foods. There's lots more information on this available, but this really has nothing to do with OP, because fundamentally OP is not about how many people Earth can feed, but rather what quality of life do we want for those people.