A long rear rack does not by itself mean a bicycle can safely carry a passenger. Before buying, check both the rack’s load limit and the bike’s maximum gross vehicle weight. These are separate limits, and a passenger setup needs to stay within both.
The complete passenger arrangement matters more than a seat cushion alone. It needs a place to sit, a handhold, foot support and protection for feet and legs. These parts should be selected as a compatible set for the specific bicycle. Different platforms, and even different generations of the same model, may have different limits and suitable accessories.
The useful buying question is not simply “how many people can it carry?” but “with which equipment and at what total load?”. Ask the seller to show the passenger guide and compatible accessories for the exact model you are considering. That makes it easier to compare the real cost of a complete passenger place, not just the price of the bicycle.