Onchange is the most important function for Odoo. similar to a negative number of seats, or more participants than seats. For example you can see below code.

        r.taken_seats = 0.0
            else:
                r.taken_seats = 100.0 * len(r.attendee_ids) / r.seats
@api.onchange('seats', 'attendee_ids')
def _verify_valid_seats(self):
    if self.seats < 0:
        return {
            'warning': {
                'title': "Incorrect 'seats' value",
                'message': "The number of available seats may not be negative",
            },
        }
    if self.seats < len(self.attendee_ids):
        return {
            'warning': {
                'title': "Too many attendees",
                'message': "Increase seats or remove excess attendees",
            },
        }

Note-The taken_seats progressbar is automatically updated.