It is standard practice within the hot tub and spa industry to quote a maximum number of people which is higher than the number of seats.
In most cases the figures quoted assume that 1 person is not seated although for larger hot tubs and spas the figures may assume that 2 people are not seated. It is perfectly possible to enjoy bathing in a hot tub or spa without being seated. However, it is unlikely that your hot tub will ever hold more people than the total number of seats.