In the mathematics of sets the cardinality of a collection of object is defined to be the number of objects in the collection, and is denoted by placing vertical bars around a listing of that collection. Following that convention we write !FOUR! = 4 or !SEVEN! = 5.
Zerger, Monte J.
"Square and Triangular Number-Names,"
Word Ways: Vol. 28
, Article 9.
Available at: https://digitalcommons.butler.edu/wordways/vol28/iss4/9