A cotyledon is the embryonic leaf in a seed bearing plant. Seed bearing is important, because not all plants are seed bearing. Ferns, for example, do not have seeds; therefore, no cotyledons.
The number of cotyledons is determined by the type of plant that the seed is produced from. Seed plants will come in two varieties. Those two types are monocot and dicot. A monocot will produce seeds with a single cotyledon, and a dicot will produce seeds that have two cotyledons.
Examples of dicot plants that have two cotyledons would be beans, daisies, tomato plants, and oak.
Maize is a monocot plant; therefore, maize seeds have a single cotyledon. Other examples from this group are other grain plants (wheat and rice), sugarcane, bananas, and onions.