1. How to get mono channels such as left-front, right-front, left-rear/side, ... of a sound card
You would want to use the SPEAKER flags: BASS_SPEAKER_FRONTLEFT for the left-front speaker, BASS_SPEAKER_FRONTRIGHT for the right-front speaker, etc. Please see the "Speaker assignment flags" section of the documenation for details.
2. if i install two sound card could i get all mono channels of two sound card
Yes, you can use 2 cards at the same time by calling BASS_Init for each of them. Also see BASS_SetDevice for setting the "device context".
3. how to play a music to a mono channel, example play a mp3 toleft-front, onother mp3 to right-front, ... abnd set volume for these channel
You can use a channel's BASS_ATTRIB_VOL setting (with BASS_ChannelSetAttribute) to control its volume level.