Does the subject want to ask "what does the mathematical primitive stand for?" Number. In algebra, the letters X, Y and Z are often set as unknowns. The number of elements refers to the number of attributes in a relational model, or the number of columns in a table, or the number of entity sets related to a connection. The number of elements is the number of attributes in the relational model or the number of columns in the table. Or the number of entity sets associated with the connection.