Do all meeting participants need a license to Read?

With seats (or licenses), you only need one if you or someone on your team wants to invite Read to a video conference to generate a meeting report, which includes a transcript, summary, recording, and meeting metrics (varies based on the plan you select).  Participants in a meeting are not required to have a license to Read.  

Another way to look at it is, if you have two people on your team that are on different meetings, and both want to invite Read, you'll want two licenses (you don't need to factor in the participants in a meeting).  If I can answer any additional questions, don't hesitate to ask.