How to create lineNum for group?

Question

There is a customer which uses BP catalog №. I have created Delivery.  This delivery contains some items with different values in ItemDecription field and with the same values in the BPCatalogNo field. I want to create a Print Template. In this template I want to group delivery lines by BPCatalogNo field and I want to renumber this groups (starting from 1, and then adding 1 for each group). I can group by BPCatalogNo, but I can't create numbering for groups.
If I try to use LineNum formula, then the numbering is wrong. I receive line numbers of delivery lines instead of group numbers.
Exactly, the number of the last row in the group is displayed.
For example, I have 3 rows:
BPCatalogNo , ItemDescription, Price
Num1                Itemdescr1          100
Num2                ItemDescr2          500
Num2                ItemDescr3          500
Expected result:
1 Num1  100
2 Num2   500
Actual result:
1 Num1  100
3 Num2  500
Please, help!

Answer 1

Yes, you are right.
Lets look at more detailed example
I have a delivery with 4 different BPCatalog Nums (Num1, Num2, Num3 and Num4)
The delivery contains rows:
LineNum  BPCatalogNo   ItemDescription Price
1           Num1     ItemDescr1      100
2          Num1     ItemDescr2      100
3               Num2     ItemDescr3      500
4               Num2     ItemDescr4      500
5               Num2     ItemDescr5      500
6               Num3     ItemDescr6      600
7               Num3     ItemDescr7      600
8               Num3     ItemDescr8      600
9               Num4     ItemDescr9      800
Expected result:
LineNum BPCatalogNo   Price
1         Num1          100
2        Num2           500
3        Num3           600
4        Num4           800
Actual result:
LineNum BPCatalogNo   Price
2         Num1          100
5        Num2           500
8        Num3           600
9        Num4           800