Ok, number of attendees is the total number of individuals who joined your room, during that session. You may have had a three hour long session, and 15 attendees came and went throughout that session, but they may not have all been in the room at the same time.
Peak Users is the highest concurrent number of individuals who have been in the meeting. So if you had 7 people in the room (including yourself) at one point but less that that at other times, the Peak User number will remain at 7, until you have 8 or more people in your room at the same time.
A session is a time when your meeting is being actively used. Once the room is inactive (no one is in the room for 7-15 min), the session ends and the next session starts once you join the room the next time. You can have multiple sessions in the same day, but if you are actively in you room for a whole day, it will be tracked as a single session.
I've seen sessions not showing up before, but I don't know why it does or doesn't show. They should be sequential from 1 to your current session.
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