operator
number of data-attribute attribute-name (of element-expression)?
The number of data-attribute operator returns the number of tokens in a list-valued data attribute for
an attribute of an element (as specified by of element-expression, or for the current element if
not specified). When the attribute does not have a list-valued type, the test will always yield a value of one.
Inside the lexical scope of an external-data-entity rule and if of
element-expression is not specified, the number of data-attribute operator evaluates to the
number of tokens in a list-valued data attribute for the current external data entity. In this context,
number of data-attribute is synonymous with number of attribute