The correct answer is: C. were
The number of guests at the party were amazing. This is because the number of guests is plural, and the verb “to be” agrees with its subject in number. In this case, the subject is “the number of guests,” which is plural. Therefore, the verb “to be” must also be plural.
The other options are incorrect because they do not agree with the subject in number. Option A, “have had,” is in the present perfect tense, which is used to talk about things that have happened in the past but have effects that continue into the present. In this case, the number of guests at the party is not something that has happened in the past but has effects that continue into the present. Therefore, option A is incorrect.
Option B, “had,” is in the simple past tense, which is used to talk about things that happened in the past. In this case, the number of guests at the party is not something that happened in the past. Therefore, option B is incorrect.
Option D, “was,” is also in the simple past tense. However, it is used to talk about singular subjects. In this case, the subject is “the number of guests,” which is plural. Therefore, option D is also incorrect.