- You need to sort the sale order lines by product id, then count the line of each product id (by python code)

google "python sort list of tuples"

a = [(1, "ahbe"), (2,"sss"), (1, "ddjj")]

a.sort(key=lambda tup: tup[0])

a

>>>  [(1, 'ahbe'), (1, 'ddjj'), (2, 'sss')]

google "python count element in list of tuples"

from collections import Counter

Counter(tup[0] for tup in a)

>>> Counter({1: 2, 2: 1})

- then edit/create the xml file, the first time of each product id, add 1 cell with rowspan.