The ratio is the one number that decides whether a course is taught or processed.
Most agencies allow an instructor more students than we take, and every centre that takes the maximum will tell you it makes no difference. Having taught both ways, it makes all the difference there is.
In the pool the instructor has to watch every student's face at the moment they clear a mask for the first time, because that is the moment somebody either learns the skill or learns to fear it. With four you can see four faces. With eight you are watching a group and hoping.
In the open water it is worse, because the students are neutrally buoyant, moving, and each one is drifting somewhere slightly different. Four is the number one instructor can keep within arm's reach on a reef. Beyond that the instructor is managing positions rather than teaching anything.
It costs us money. A busy week turns bookings away that a larger ratio would have absorbed, and we have watched people walk down the road to a centre that could take them the same day. That is a decision we made on purpose and it has not changed in nine years.
The other half of it is the fifth day. If a student needs another morning in the pool, they get one and nobody pays extra. A certification handed to somebody who is not ready does not stay behind when they leave. It follows them to the next centre, and to the one after that.