The knowledge sources do not explicitly address what happens when a single day contains more than 100,000 events and no limit parameter is specified.
What the documentation does confirm is:
The limit parameter (which caps results at 100,000) is optional — if you don't set it, there is no mention of a default cap being applied.
The limit parameter description states: "Use this parameter if you want to limit the max number of events to be returned. Value cannot be over 100,000." [Export API docs]
This suggests that the 100,000 cap only applies when you explicitly set the limit parameter. If you omit it, the API may return all events for the queried date range. However, I cannot confirm this with certainty based on the available knowledge sources.
I'd recommend checking the Mixpanel Raw Event Export API reference directly or reaching out to Mixpanel support for a definitive answer on this behavior.
