A1=1 B1=1
A2=2 B2=""
A3=3.... B3="".....
A20=20 B8=2
B9=""
B10=""....
B16=3

Strictly speaking that's not every 8th row. That aside, try something like

=IF(ROW()=1,$A$1,IF(MOD(ROW(),8)<0,"",INDIRECT("A "&(ROW()/8)+1)))

HTH,
Andy