If you uncheck 'Event->Edit registration->Participants can change answer later', the questionnaire can only be completed once, after which the answer button is hidden.
You can set a custom maximum number of companions under 'Event->Participants'->Action->Change Response'. You can also import these values by adding a 'companions_max' column with the respective values. The import must be done via 'Event->Participants->Assign Participants->Import List', as this value is event-specific.