In the book, Jay approaches him. Then he says to himself "SAY! Look at his fingers!". Then he counts how many fingers he sees. But then he asks himself, "How many fingers do I see?". He counts them (in the video, the Eleven Finger Creature's fingers get erased as if he made a fist). As Jay counts, the fingers are erased, then they appear one by one during the count. Finally, Jay says, "HE HAS ELEVEN!". Kay is excited about how new this is (saying, "Eleven!? This is something new!"). Then she wishes that she had 11 fingers too.
We also see him in Dr. Seuss's 123, where his fingers are counted.