You can implement a joiner between Table B (as master) and Table A (as detail), the joiner type must be detail outer, so you will get all rows in table B and their equivalent in table A. Then you could obtain those rows in table B wich doesn't have an equivalent in Table A using a filter transformation. And finally you can count the rows using an aggreation transformation.
Table A
Joiner A-B --- Filter (a is null) -- Agregator (count rows)
Table B