what is needed for an interesting realization, technically spoken

when inviting to join, you have to ask, what type of sound device each of the musicians/djs/realtime composers is going to use: vinyl, cd, cdrom, files from a laptop, usb-stick, tapes, drum machines...
i have no preferences, but i need to know in advance in order to be able to have enough equipment; each participant also has to say, what s_he can bring and what we have to find; sometimes it is also possible that for example people share a mixing desk etc.

all these single signals have to go into one main mix (that means we need a lot of cables here as well) and the general PA-system (amplifyer and loudspeakers), and from there the sound is spread over the space again. a sound technician is necesary.

as experience shows, a lot of people like using laptops - this means, we need DI.boxes, to eliminate noise from the soundcard.
and you have to think of the electricity needs: enough cables and enough power in the space. it is not about having the fuses go all the time :-)
and we need enough tables and chairs for the participants.

number of participants

this only depends on the amount of technical equipment you can find.
and how willing they are to share it. there is no conceptual limit.

we need one meeting before the performance in the space of the performance, in order to set all technical stuff up, best on the same day, but with some time between setting up and performing ; this is also when i explain how the system works - we will not do a rehearsal.

light situation

it is necesary that everyone can see what s_he is doing, so for turntables and cd-playing some little table light or something would be nice; for the overall situation the "slide show" is what we will all see, apart from that - no special needs, if there is something to let it look nice at the space, fine, otherwise we will just do without. the slideshow can go to the wall, or if there is nothing to project on, a screen (can be improvised with bed linnen, if there is none).