As a rule, we typically fund ALC classrooms at a 20:1 ratio, as many of our classrooms are not designed for larger class sizes. I believe that this is the price point at which we can balance revenue and expenses as well. Most of the time our Evening HS and S-Term classes are larger (25-30) initially, simply because we always have "no shows" that enroll but never attend. These aren't policies, just present practices.
The information you seek may depend on what type of program you wish to use. Class size may depend on if you are using "independent" classes with different students each day, a full five day program with the same students each day, part time students, etc. Our independent program works well with about 25 to 30 students each day with different students each day. Each teacher has about 5 to 6 students per block in a block schedule.
The National Alternative Education Association lays out a recommendation of a 12:1 student to teacher ratio