Don’t think it’s possible to have different document numbers per LU. What’s the reason you need this? 
You could setup different Number Counters per document class, and then setup different default document classes per LU, but that would require a different Doc Class per object.
 

No, this is not possible today. A fundamental problem is that the document must be created before you can connect it to an object. This stops us from doing clever things using custom events in this area.

We have some client support for controlling some default values on new documents created “from certain objects” though (class, format, language...) under Document Basic / Document Defaults per Object.

Unfortunately we did not include number generation as part of those default values. It would be natural to extend that functionality to also include number counters or booking lists, but there are no plans to do that now. If you want to you can file a support case, which will be turned into an idea wall request, such that we have this idea registered.

Like André, I am also interested in understanding why you want to do this.

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Thanks!