For preschool, it's 11:1
As a teacher myself, 16:1 sounds awesome. So, I looked it up for where I live. It's 17:1 in CO. This must be for adults in the school: students in the school. I always had at least 25 kids in a class by myself.
This ratio tends to be used as a proxy to measure a number of things. For instance, a high teacher/student ratio means a single teacher must pay attention to a lot of students, which probably implies that the teacher can't really pay attention to all the students and the quality of the teaching experience may suffer as a consequence.