One connection for one SAP BW server to access any number of BEx query and InfoProvider from Dashboard. But you have to create Query Browse's query for each BEx quey or InfoProvider Data set.
It is better to create Dashboards in SAP NetWeaver BW if there is only SAP BW data consumed by Dashboard. But SAP NetWeaver BW Dashboards will be access from Portal (if published on portal) or from SAP BW, it is not available on SAP BO Launch Pad.
Thanks~