Why are human embryos usually produced one at a time while in plants, seeds are produced in large numbers?
Consider the complexity both humans and plants. The energy and needs required of the host to successful reproduce the life form will be vastly different based upon the life form. There is no doubt that humans are a more complex life system than plants, and so to bring a human life to a viable point will require more of the host. That would be one reason why most (not all) pregnancies produce one embryo. Singular pregnancies tend to produce healthier babies. Look, for example, what happens when science intervenes on behalf of infertility and multiple fetuses occur. The babies are generally born earlier at smaller birth weights with less developed organs. In addition, outside of birth complications and potential illness, there is really no natural predator of a human baby.
Plants on the other hand are less complex life systems, and it does not requires less to successful develop. Thus more can be successful produced at one time. In addition, plants have natural enemies that could deplete the number of successful reproductions. Consider drought, erosion, flood, and wind. What would the plant kingdom be like if only one fruit was produced and this single life dried up in a drought? In addition, animals frequently consume some of the seeds, so the plant species would die out if only one seed was produced.