3rd column billing document 4th no. of items for a billing document

Are you looking for another column with the list of Billing Docs but unique numbers only (removing duplicates) and a column next to that with the total number ofitems for each unique doc?

If so, you can copy the first column into another then remove the duplicates (Data -> Remove duplicates).
Next, add a formula similar to this in the cells next to the resulting list:

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Where D2 is the Billing Doc. number.

I'm sorry, I don't understand how the data I'm looking at relates to what you are looking for.

The Document Number column has a large number of duplicates. Do you want to remove them or no?
"in column G is it single line item or multiple line items" <- Unsure what this means.
"in column H tax code" <- This suggests a one to one relationship to the original data (as does having it in the next column), if this is the case then what transformation is required.

Sorry if I'm being a bit dense but I require a few more words required to express the logic of the change you wish to see for me to understand.