"Is" is the correct verb. This question has to do with subject-verb agreement. In the first part of the sentence, "A huge number of windmills," "number" is the subject and "Of windmills" is a prepositional phrase. The subject must always agree in number with the verb. So, the singular subject "number" takes the singular form of the verb, which in this case is "is." Many people make the mistake of choosing the verb that agrees in number with the object of the preposition, which in the above example is "windmills." Just find the main verb of the sentence and choose your verb accordingly.