Plants with ovaries having only one or a few ovules are generally pollinated by

Wind pollination is a chance event, so wind-pollinated flowers produce enormous amounts of light, non-sticky pollen and have well-exposed stamens and large, feathery stigmas.
Such flowers often have a single ovule in each ovary and numerous flowers packed into an inflorescence, e.g., the corn cob, where each grain develops from an ovary with one ovule.
Having one ovule per ovary suits the low probability of many pollen grains landing on the same stigma.
Flowers pollinated by bees, butterflies and birds usually receive many pollen grains in a single visit and commonly have ovaries with many ovules.
Hence plants with ovaries having one or a few ovules are generally pollinated by wind.