The word fearlessness contains one diphthong?

You are correct! The word "fearlessness" contains one diphthong, which is "ea" in the first syllable.

A diphthong is a combination of two vowel sounds that are pronounced together as a single unit. In "fearlessness," the "ea" sound is a glide from the "ee" sound in "see" to the "ah" sound in "father."