21. Prime Minister Turtle ordered the shrimp soldiers and crab soldiers to stand in a solid square formation facing the Dragon Palace. Little shrimp Nini has 4 people directly in front of her, and 3 people directly to her left; Little crab Momo has 3 people directly behind her, and 4 people directly to her right; There are 7 people between Nini and Momo (in a straight line). The entire formation has $\qquad$ people.
256