I know you were after a number, but class size and OKness also depends on the capability of the teacher.
DD1 has an excellent teacher. She had her last year too. Last year there were 22 kids in her transition class (kindergarten), this year with the other teachers in DD's year, several parents have asked to have their child transferred into this teacher's class. Currently there are 27. She handles it with ease and even though it's a composite class, no child slips through. However there are only 15 in another teacher's class in the same grade, and from what I have witnessed, that is too many for her.