That is fine, if you have a midi controller keyboard that plugs into USB. You only need an interface with 5-pin midi (in/out) if you have older type of midi connector.
> but if ur using 8 mics then sureley u just need 1 output into the pc soundcard ??
The 8 mics will go into 8 separate tracks. As for the output, you do not route it to the PC soundcard, rather, the stereo out goes to your powered monitor, or the PA system.
As for multi-outs, they are all optional. For example, you may have a second or third pair of monitor, or for individual musician headphone monitoring. Another use may be for surround sound mixing.