Your decision should depend in great part upon the skill level of not only the shooters but of your skill level as an instructor. I have supervised up to five shooters at a time. They all, at the least, had the basics down and, I had lodes (or loads depending on how you want to look at it) of experience and years of training. When my shooters shot tactically it would be one on one. When they shot anything outside of the normal the most would be two (shooters) to one (instructor). Problem shooters or brand new shooters would never be more than two (shooters) to one (instructor) if I was the only instructor present.
Of course the range set up also has something to contribute to how many shooters you can effectively manage. If this is an indoor range with actual divided shooting lanes (at least dividers back on the firing line) this could mean 1 on 1 only. This would be because if you get in too close to shooter on point A, you will no longer be able to glance over and see shooter on Point B. If you are able to stand close enough to either shooter to give up close instruction from over the shoulder or by the side with still being able to glance up, or over, and see the other shooter then you are probably OK safety wise. This, of course, as long as you are qualified enough as an instructor. Note by qualified I mean that you at least have quite some time instructing other shooters while doing it right and, while doing it safely. Of course firearms instructor certification (NRA, military, police) would probably be much better than just self trained or trained by a shooting pal.
If you have any doubts at, all no matter how small, then 1 on 1 is best until at least the point where each shooter demonstrates a fairly good proficiency with handling firearms in a safe manner both while shooting and at all other times.
By the way, never have other people crowded around the shooter and yourself while training. Other shooters must wait behind the firing line, and must keep quiet or at least not interfere with the instructor and the shooter(s) who are actually on the firing line. Good range discipline is important to prevent distractions that can contribute to accidents.
Also, never assume that anyone who is new to firearms will have enough firearms sense to handle them in a safe manner such as by always pointing the in a safe direction, etc. To do so could be fatal. That would ruin all of the fun! Safety at the forefront allows for many years of shooting fun down the road. Please don't forsake range safety just for the sake of having fun. Instead, make the safety lessons part of the fun.
All the best,
Glenn B
