Hi Patrick,
What counts is the number of base InfoProviders / PartProviders which are queried simultaneously. Here the recommendation is still approx. 10. If it becomes more, then it will turn out to become counter productive. We have to realize that also other hardware and software partitioning mechanisms are used which can generate a massive number of parallel queries. To present the result, all sub queries have to be consolidated. That's why it is still a best practice to limit it.
Please also refer to SAP Help.
In this context it is important to consider query pruning, e.g. by defining constant values or the MultiProvider Hint table. Please refer also to SAP Help.
Last but not least, please note that for SAP BW on HANA the MultiProvider becomes obsolete. It is replaced by the new Composite Provider.
Best regards,
Sander