Question

why most plants produce a large number of seeds

Answer 1

From my point of view,
It's a matter of survival. Only a small percentage of seeds produced by plants will actually grow into mature plants. Some won't germinate. Some won't wind up in a place where they can germinate. Some will be eaten. Plants produce so many seeds so that enough of them will grow into mature plants to ensure survival of the species

Answer 2

most plants produce a large number of seeds because most of the seeds don't get dispersed and some of them don't grow. After all these reasons there are less chances for the growth of a seed. Therefore some plants produce a large number of seeds so that one day some of these seeds will grow into a plant.