Try either one of these
First one counts ALL Rows in your table (including blanks)
Grand Total = CALCULATE ( COUNTROWS ( TableName ), ALLSELETED ( TableName ) )
the second counts all except blanks
Grant Total = CALCULATE ( COUNTA ( TableName[Column] ), ALLSELECTED ( TableName ) )
Take your pick according to your needs!
just FYI and reference
COUNTROWS(Table) = COUNTA(Table[Column]) + COUNTBLANK(Table[Column])