To roll it out, consider explaining why you're requesting cameras on (in student-friendly terms: "This lets me know how the lesson is going for you and/or when you want to participate" etc.). You might also add that students who have extenuating reasons/circumstances for not having cameras should check with you outside of class. After you roll out the importance of having cameras on (sharing the "why" in student-friendly terms), I might reinforce at the beginning of each lesson with a gentle reminder: "I see most of your smiling faces. Just need a few more cameras on." If a few cameras continue to remain off, maybe gently nudge with a private chat to ask if everything is OK or follow up with that student another way. If you're concerned students might be anxious about people will be able to see their surroundings, consider teaching students how to change their virtual backgrounds. The benefits of reinforcing "cameras on" early can be huge, and the downsides of not doing so can be considerable (on relationship-building, classroom energy and culture, students' attentiveness, etc.).