What ratio will be a perfect one for a conducive classroom, for the teacher and student?

For the purposes of student engagement a ratio of 1-15 is ideal. This allows for effective group work and question and answer sessions.

It also allows for differentiation and the ability to pair differing levels of students together to enable peer support.

The smaller the classroom the better, especially if you have weaker students amongst your class. It is easier to manage, you are able to give your students more attention and you can determine the points of weakness and focus on them rather than just teaching the curriculum without adjusting it to your students' needs.

For a perfect bonding and proper learning among students the best ratio is 1:20