A series of events I participate in annually requires a 1:1 ratio for all shooters, regardless of ability or previous experience. Extra insulation for promoters, and very very effective for the students. Others range from 5:1 to 3:1, depending on venue, sponsor, or insurer requirements.
I’ve been to several that are 10:1 or slightly more. Sub-optimal, but how much so depends on what you’re doing and with whom.
In fact on a large group I will usually have a 5:1 / 6:1 max ratio of Cadre and if the class gets to the 18 student count I will actually bring on 1 additional person who is strictly the Range Master with no student responsibility. They strictly control range operations/ Safety.
That’s a luxury we have and NOT common to many programs I have attended.