Norm,
It has been a while since I was in the military but it is what we used to call “span of control.” The width of the span is dependant on a couple things. The maturity/experience/aptitude of the person and the maturity of the organization. The more experienced and/or mature the wider the span can go with less risk.
When we are training a BB or MBB we do not have anyone mentoring under them so they can focus on training.
Normally we begin at a 1:4 ratio when we have tight control. We do this in the beginning od a deployment – because the organization is typically trying to adjust or figure out how to internalize what is happening. We also start a new BB here to figure out if they are any good at mentoring. There are people who are just poor mentors and they should have 0 (check out the book “Break all the Rules” about trying to fix people). There are people who are mediocre belts (not necessarily a bad thing) but are fantastic mentors. They get up to but no more than 10 (as Adam said).
MBB’s we begin at 1:4 because they have BB experience and should not be struggling with the training that badly. You may stress them when they mentor, train and do a project but if they are going to break you might as well do it before you get a lot of money invested in training them to do something they can’t do. They should be able to run up to 1:10 as well but we never really stretch them that far. We stay around 5 or 6:1 and have them spend more of their time mentoring up into the organization i.e. GM’s, VP’s, etc.
You basically have the “it depends” response but I hope that gave you some help in deciding what the correct levels would be for your organization.
Good luck
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