I don't know the lx20 but it looks like it has separate record arms for all 8 line level inputs.
So, if you want to use several microphones at once, you either need an 8 channel preamp with line outputs per channel,
or a small mixer with at least 8 discreet mic preamps and corresponding line outputs.
If you just want to record a guitar (via cable), you'd need a DI box with line output, or a preamp with instrument level input and line output.
You'll also need a way of listening and again, the unit only has line level I/O so if you went with the mixer option you could route the lx20 back to the mixer and let it provide you with outputs for active speakers or an amp and speakers, a volume control and probably a headphone output too.
Looking at it, a 10 channel mixer is probably the best way to go about it.
Maybe someone else has ideas?