I found this in the UK's Air Navigation Order - Article 20:
(7) (a) This paragraph applies to any flight for the purpose of public transport by an aircraft registered in the United Kingdom:
(i) on which is carried 20 or more passengers; or
(ii) which may in accordance with its certifiate of airworthiness carry more than 35 passengers and on which at least one passenger is carried.
(b) The crew of an aircraft on a flight to which this paragraph applies shall include cabin crew carried for the purposes of performing in the interests of the safety of passengers, duties to be assigned by the operator or the commanfer of the aircraft, but who shall not act as members of the flight crew.
(c) (i) subject to sub-paragraph (ii), on a flight to which this paragraph applies, there shall be carried not less than one member of cabin crew for every 50 or fraction of 50 passenger seats installed on the aircraft.
I read this as meaning:
More than 35 seats = Must have cabin crew.
Twenty or more passengers = Must have cabin crew.
So an aircraft with 34 SEATS and 19 PAX does not need cabin crew.
But as soon as it hits either 35 seats or 20 pax, you need one CC per 50 (or fraction of) seats.
But of course, it only applies to UK registered aircraft - and a reduction in cabin crew is allowed if a reduction in pax numbers and their distribution throughout the aircraft is agreed with the CAA.