dgachet The red LED is the power indicator. If it is steady your power supply (and the connecting cable) are sufficient.
I didn't have enough opportunities so far to estimate the maximum number of Wi-Fi clients per MoodleBox. The maximum I had was somewhere 13 participants - but in a very busy application. The students competed in an Active Quiz https://moodle.org/plugins/mod_activequiz. A Pi 3B (not B+) managed it, with one hiccup. Initially I thought we had exceeded the processing power of the Pi. But it could have been an undervoltage. I was not careful about the power pack then.
I'm also keen on finding this number, with Pi 3B and 3B+, since I will face this situation soon! I thought 25 being mentioned here (by the developer?).