Sometimes they will. Sometimes they will not. A vehicle advertised for four passengers may have nowhere near enough boot space for four large cases, four cabin bags and a folded pushchair.
Count the shape, not only the number
Tell the operator how many large hard-shell cases, soft bags, cabin cases, pushchairs, wheelchairs, golf bags or other unusual items you have. A photograph can help when capacity is close.
Do not plan to fill every seat automatically
An eight-seat people carrier can still have limited luggage space when all seats are occupied. A trailer or larger vehicle may be needed for a large group, and this must be arranged in advance.
Allow for the return
Holiday shopping can increase the load. Make sure the return booking uses a vehicle with the same or greater capacity.