Does labelling the prompt number on each essay for AP exams really matter?

I forgot to label them, and I think that was one of the reason why I got lower score than the score that I was expecting to get. AP English exam for this year wasn’t too difficult, so I don’t know what happened to my score.

It’s not the reason for your lower score. Labeling the prompt makes the grading go smoother, but the readers will figure it out eventually.