It's definitely total persons on board including crew. The guide to completing a flight plan form (
CAA link here).
Item 19 - Persons on Board
Insert the total number of persons (passengers and crew) on board, when required by the appropriate ATS authority.
Insert "TBN" (to be notified) if the total number is not known at the time of filing.
Even if you haven't filed a flight plan, the verbal request from the tower would have the same meaning, so it's total on board. As it's for SAR purposes, if your pax died enroute then I wouldn't amend the number, when they fish bodies out of the sea, they won't know which ones were dead or alive beforehand, they'll just be working to a headcount
