The sentence you provided illustrates the use of the past perfect tense, which is formed by combining the auxiliary verb "have" in its past tense form, "had," with the past participle form of the main verb. In this case, the main verb is "have," and the past participle form is "had." The past perfect tense is typically used to describe an action that occurred before another past action or a specific point in the past. In your example, "Harry must have had more haircuts" suggests that Harry had a larger number of haircuts in the past than the other boys in his class. The past perfect tense is used to emphasize that his accumulation of haircuts had happened before the rest of the boys had theirs.