To sign line, as in a queue of people, form your hands into the ASL number 4 sign. Center them in front of you, lined up such that your left hand's pinkie is touching your right hand's index. Then separate the hands while keeping them in a straight line. Your eight fingers represent the big number of people lining up.
Teach your toddler the line sign when you have to queue up at a carnival, zoo, or museum.
This is a useful flash card taught best in conjunction with other place flash cards where long lines of people normally form. Then bring a backpack of goodies with their stroller to keep your toddler sufficiently distracted: their sippy cup or bottle, teddy bear, picture book, rattle, snack, coloring book and crayons, and so on.