That depends on what kind of plant you are talking about. Are you thinking of flowering plants?? In some cases a plant can grow different sexes of flowers depending on the environmental condition but only in certain species.

Many plants have both sexes, and many are unisexual. If you need two of a species if may be that the plant could be either sex (like hazelnuts). Those which have both sexes may still not be self-fertile and need another plant to fertilise each other (apples) or they may be self- fertile. Yes some plants may change sexes. Arum (jack in the Pulpit changes sex- male when conditions are stressful, and female when there is lots of water and fertile soil