A sentence with one blank is given, indicating that something has been omitted. Choose the word that best fits the blank appropriately. As the festival approaches, the number of customers _________

The given sentence is in simple present tense and states a general fact, hence the latter half of the sentence with the blank will also be in the simple present tense. Since the subject in the latter part of the sentence is singular .i.e. 'the number', the verb will be followed accordingly. Hence, option 4 is the correct answer.