Trying to create a dynamic row number in a table visual

Two points if known, can provide accurate syntax.

Use case: you are trying to get the row number based on filter selection of each row. 

Assuming you have index column to identify unique row, then you can use this.

Try this:

Row_Number = 
CALCULATE (
    COUNTROWS('Table');
    FILTER ( ALLSELECTED ('Table'), 'Table'[Index] <= MAX ( 'Table'[Index]) )
)
 

Or  If not you can use all the filters that are applied to the table.

Row_Number = 
var _v1 = SELECTEDVALUE ( 'Table'[filterColumn1])
var _v2 = SELECTEDVALUE ( 'Table'[filterColumn2])
RETURN CALCULATE ( COUNTROWS('Table');
    FILTER ( ALLSELECTED ('Table'),
           'Table'[filterColumn1] <= _v1 && 'Table'[filterColumn2] = _v2 )
)

Hope this helps!