How much pollen is needed to pollinate a flower? Assuming 100% of the pollen gets delivered to exactly the locations it needs to pollinate a female flower, how much pollen is needed to pollinate a flower? If it's more than one unit of pollen, what, if anything, prevents more than one plant's pollen from pollinating the flower? If we need an example plant to answer this question, let's assume zucchini. For instance, if you plant a zucchini in the middle of a spaghetti squash and an orange-fleshed squash, is there any likelihood at all that you'll get a single zucchini pollinated by both in one generation (such that the next generation of cross-pollinated zucchini would be both stringy and orange-fleshed instead of just one or the other)?

A single ovule in a fruit is pollinated by a single grain of pollen. So, in theory, a fruit with n seeds can be pollinated by n grains of pollen. In reality, of course, not every pollen grain makes it to the ovules, but if we treat your assumption as true, then a flower can be fully pollinated by as many pollen grains as there are ovules in the flower.

If you plant your zucchini and cross your zucchini flower with pollen from spaghetti squash and orange-fleshed squash, you will end up with a zucchini fruit filled with seeds that are either half spaghetti squash or half orange-fleshed squash. If you planted the seeds from that zucchini, you would end up with some individual plants that have spaghetti-squash traits and some with orange-fleshed squash traits. You would not end up with any plants with both stringy and orange-fleshed fruit.