Hippos go Berserk! by Sandra Boynton

From chapters.ca:
This offering from the inimitable Sandra Boynton is a wonderful counting lesson embellished with intrinsically funny hippos. One hippo is all alone, so he calls up two others, who come over...followed by three hippos, then four more, and so on until there's a party of hippopotamic proportions in the living room! But when the party's over, the hippos depart in reverse order, a group of nine first (plus one unexplained beast guest), then eight, seven, and so on until the first hippo is alone again and missing his 44 friends. The jaunty rhyming text sets up some great visual gags. This is a truly fun and inventive book.