Hello JensT,
Assuming the data is on Sheet1 & starting from A2 to down & across. also you are on Excel 2007 or later, since you have >256 columns
In a new sheet eg: Sheet2!A1 enter
=COUNTA(Sheet1!A2:A100000) `this will give how many rows have data (assuming there are no blank cells between data)
B1,
=A1*COUNTA(Sheet1!A2:WZ2) `this will give the count of total data (assuming there are no blank cells between A2:WZ2)
Then in A3 and copy down as needed.
=IF(ROWS(A$3:A3)<=B$1,INDEX(Sheet1!$A$2:$WZ$100000,MOD(ROWS(A$3:A3)-1,A$1)+1,INT((ROWS(A$3:A3)-1)/A$1)+1),"")