Each item can span from 1 to 12 rows

Row span works as long as you have multiple rows. For example, if you only have 2 rows of content, you cannot have an item to span 3 rows because the highest you can get is 2.

3 rows 6 rows 9 rows 12 rows
Row
Row
Row
Row
Row
Row
Row
Row
Row
Row
Row
Row