Oh boy... get ready for a ridiculous flood of replies on this one..
My take, and I am certainly no expert, is that you will never find a baseline answer. It's like the Drake equation right? There are sooo many factors. My personal guideline is taking into factor the class of the water, the experience of the MAJORITY of the boat, the expectation of danger vs fun among potential crew and of course the inevitable alcohol question, which all leads into the question "Will/can this crew work together to get us through anything that we encounter?"
I've never been a guide, but I do recite an informational speech before each venture out. Make sure people know where/how to sit and how to paddle, use a throw bag, highside etc. After all you are the leader and need to own up to the job. After that, you're really giving yourself up to the river gods. Think about how many times you've run a line, but today it was a little different and caused all kinds of chaos, or the exact opposite, you did something different and it was easy as pie? In the end, you're gut tells you what you should and shouldn't be doing.