The main question is whether the synchronized controls remain usable when twenty devices are running at once. A feature that copies taps across several phones won’t help the lesson if the secondary devices lag behind or an icon appears in a different position. You can read a test of DuoPlus Cloud Phone here: https://gologin.com/blog/duoplus-cloud-phone-review/ . The review checks its synchronizer with several phones, along with startup speed, proxy requirements, and usage charges. Those results should help you decide whether it can support a whole class or only a smaller demonstration group.