Additional features, settings, or issues not covered by specific Microsoft Teams categories
The best you can do here is get the attendance report, if it was enabled for said Live event, as detailed for example here: https://support.microsoft.com/en-us/office/get-an-attendee-engagement-report-for-a-teams-live-event-b3101733-2eda-48a6-aeb3-de2f2bfecb3a
While it will not give you directly "the maximum amount of people that attended a Live Event at one time", you can work with the data within the report to come up with an estimate. Unfortunately, if the option was not toggled when the event was scheduled, the report will be unavailable.