Hi Patrick,
As such there is no limit for the maximum number of infoproviders to be included under one multiprovider. But for the optimal performance this number should not exceed 10.
Though HANA provides tremendous speed for loading of the data due to the in memory columnar data storage, the basic principle remains the same.
SAP Help says:
Technically there are no restrictions with regard to the number of InfoProviders that can be included in a MultiProvider. However, we recommend that you include no more than 10 InfoProviders in a single MultiProvider, otherwise splitting the MultiProvider queries and reconstructing the results for the individual InfoProviders takes a substantial amount of time and is generally counterproductive. Modeling MultiProviders with more than 10 InfoProviders is also highly complex.
http://help.sap.com/saphelp_nw2004s/helpdata/en/52/1ddc37a3f57a07e10000009b38f889/frameset.htm