As far as I know you can't. You would need to make several "Line Item" fields (Line Item 1, Line Item 2) and link in the document that way. At least then you can ask InDesign to ignore the empty fields. Eg, if one order has 10 line items and another only has 1 line item, the 9 blank fields are ignored. This means setting up your template for the most possible fields.
However, having said all that, there might be a script for it!
Thanks for the reply. I'm having a difficult time with this, trying to think of some other software that might do what I'm after. Looked at Crystal Reports, but I don't think it will work either...
Check out emsoftware.com - either InCatalog or InData sounds like it might be overkill for your purposes, but those are the kinds of tools you use when you need to make a non-simple document based on data housed outside of ID in a spreadsheet or database or whatever. There are a lot of plugin products like that out there for data-driven publishing in InDesign.
Alternately, if you know a bit of VBA (and assuming that said CSV is spat out of an Excel file), you could write a script that would break out your duplicate order numbers programmatically into new cells. But, basically, Data Merge won't do what you want, so you either need a better tool for getting your variable data into ID, or some way to massage your data outside of ID so you can use Data Merge.
InDesign's data merge isn't really a query application such as MS Access. It can place text from one field at a time to placeholders inserted into an indesign file, but it can't intelligently parse the file, and certainly couldn't create one to many relationship results on an order. the database relationship in indesign is strictly one to one.