The number of boxes sold leapt from 19,400 to 33,542.

In the sentence 'The number of boxes sold leapt from 19,400 to 33,542', the word 'leapt' is the past tense of 'leap' and describes the action of the number jumping/increasing dramatically. It serves as the main verb of the sentence, expressing the action. Therefore, the correct word class is verb.