It's hard enough to get bands (who should know this stuff, Oh! We have an acoustic guitar too, where do we plug it in?) to say what they want.
I would think something like a stage plot that asks to list all the people who will be doing something, where they will be standing and what they will be doing. Maybe they draw a circle with a number and below list who that number is and what they do. Somehow you have to get them to think in terms of their production and everyone that is involved in a performance. Most folks will have done some sort of rehearsal and can visualize everyone standing on stage. Along the lines of Glen's narrative of the performance. That way you know that one person will be standing at a lectern playing tracks from their phone and showing slides on a projector, or 5 people will be strumming acoustic guitars with one keyboard and 7 singers.
Just asking them how many microphones they'll need and how many DI inputs they'll need will invariably end up leaving something out.
I remember an event where I had a list of everyone in the band but that day was told that some of their students would be doing a few songs opening up. And no idea how many doing what until they wandered out of the crowd onto the stage looking for a place to plug in. Initially it was that the students would play their instruments. Okay, fine, everything is set up for that. Only to have 4 or 5 additional kids come up with electric and acoustic guitars and expecting to sing.