If you enter your row number in cell B1, then

=COUNTA(INDEX(1:65536,B1,1):INDEX(1:65536,B1,256))

will give you the intended result. This solution avoids
the INDIRECT() and the OFFSET() functions which are
volatile (see
http://www.whooper.co.uk/excelstuff.htm#tip1
).

HTH,
Bernd