Is it possible to assign various MIDI channels so that each plays a different instrument within one instance of Rapture Session or do I need to load a new instance of Rapture Session for each different instrument.  Cakewalk TTS allows you to assign 16 different MIDI Channels for example

No, use multiple instances of Rapture Session. Rapture Session is not multitimbral. Rapture Pro has a multitimbral option; mapping the first 6 MIDI channels to the 6 RPro elements.