How can a woman make up the prayers and fasting that are obligatory to be performed after she learned about the necessity of ghusl (ritual bath) due to a wet dream?

You are obligated to make up all the prayers you performed while in a state of major ritual impurity (janabah). If you do not remember their exact number, then strive and investigate to ascertain what will absolve your responsibility before Allah. As for fasting, it is not affected by a wet dream. The wet dream that necessitates ghusl for a woman is that which is accompanied by ejaculation.