If I understand what you want to do correctly, you don't need the OFFSET
function. If you are starting you block counting in Row 1, then use this
formula and copy it down...

=1+MOD(ROW()-1,20)

If you are going to start in a row other than Row 1, then change the -1
to -rownum where you would use the starting row number in place of rownum.
So that, if you were starting your count in Row 4, then you would put this
in that cell and copy down....

=1+MOD(ROW()-4,20)

These formulas will preserve the correct numbering if you insert rows later
on (although you would have to copy the formula from one of the adjacent
cells to preserve the continuity of the formulas).

Rick