QandR counts the number of unique participants per month, regardless of the number of sessions you organise. If you organise a larger project with multiple sessions within one month, and the same person participates in two sessions, this person is counted only once, because for that month this is only one unique participant. Technically, QandR looks at the unique phone and does so within a specific browser. So if you have a participant who logs in with a Chrome browser one session and logs in with a Safari browser the next session on the same phone, QandR will count that as two participants. But this is a scenario that will not happen often.