In an ideal situation (and I've done this countless times) you have an L1® for each performer. Here's a video that explains why The L1® Approach and the History of Amplification
As a practical matter, can have two or more performers share an L1®.
Because these units are not cheap, I guess my next question involves how much L1 does each person require? Meaning, does the Bass really need a full system if his frequency response is so limited.
If the bassist is also a vocalist, then there are great benefits for the bassist to have his own L1®. Whether or not you devote an L1® to bass guitar, it's nice to pull a high pass line out of the bass amp and run that into an L1® close to the bassist. This adds great definition to what he's playing.
Would he even need a ToneMatch for his set up or just an amp simulator or maybe even just direct?
I've done it all three ways. There are some ToneMatch Presets for bass that sound great, but some bassists want to build their tone just like guitarists.
For the keyboard set up, I find that live keyboards come to life with all of their inherit stereo sounds, especially for Leslie simulation and such. Would you have to have more than one unit or is the experience of playing through a single unit palpable because of the 180 degree line array?
I don't usually run any sound sources through more than one L1®. You lose the Cocktail Party Effect.
Sometimes I make an exception for keyboards if it's important to have stereo effects, I run one keyboard output to an L1® close to the keyboardist, and the other keyboard output to another L1® across the stage (someone else's L1®). Unless I the keyboard stereo effects are the feature of the band, I wouldn't dedicate two L1® to keyboards.
Vocals seem like a no brainer.
If you haven't already done it, check out Cocktail Party Effect <---
This is why I always run each vocal to just one L1®.
How are we doing so far?