snippet from reference
The term cardinality actually has two different meanings depending on the context of it’s usage
one meaning is in the context of data modeling and the other meaning is in the context of SQL statements. . . when the word cardinality is used in the context of data modeling, it simply refers to the relationship that one table can have with another table.
In SQL, the cardinality of a column in a given table refers to the number of unique values that appear in the table for that column. So, remember that the cardinality is a number.