The head of a sunflower is actually hundreds of smaller flowers working together to attract pollinators. Each large yellow petal is its own individual flower, and the bits in the middle are tiny five-pointed flowers if you look closely.
Sunflowers share this property with the rest of the family Asteraceae, which includes daisies, dahlias, dandilions, marigolds, chrysanthemums, and so many other similar-looking flowers that gardeners sometimes refer to unidentified members as DYCs, or ”damned yellow composites“.
The heads of these plants can be called an inflorescences, which is the word for a cluster of flowers regardless of whether the cluster looks like a flower or not.