Mixpanel recommends using Session Start events as a proxy to estimate the number of replays, particularly if you use timeout-based sessions:
"The Session Start events are a way to estimate the rough amount of replays you might expect. This is especially true if you use timeout-based query sessions. However, because our sessions are defined at query time, we cannot guarantee these metrics will be directly correlated." [Session Replay FAQ]
To measure Session Start events per user in Insights, you can use the Frequency per User measurement on the Session Start event, which calculates the number of sessions per user. [Sessions]
In summary, use Session Start events (not $mp_session_record) as your metric for estimating session recordings per user, keeping in mind it's an approximation rather than an exact count.
