If you rent our boats for us doesn't matter how many persons are on boat till match is capacity. MS FANNY is max. 150 pax (with crew) MS KLARA 40 pax (with crew). Consider that it is maximum capacity and doesn't mean that would be comfortable if is full. That's depend of event type.
You can be alone on a rented boat, or only you and your partner, or any as you want. That's not change the price.