Panelists are the speakers in a Zoom webinar. Zoom explains the panelist numbers in this Zoom support article

Zoom Meetings do not have panelists/speakers, only a general capacity limit. 

Please double check your event data and ensure that you do not have more panelists (speakers of parent and sub-sessions combined) than your allotted number. Please count all the speakers associated to your parent sessions and also all speakers in subsessions (presentation level).