Seat struts increase the weight transfer from the seat. They stiffen the rear, but they also allow more leverage for lifting the inside wheel. Essentially you’re going to get harder “dig” or bite on the outside tire.
In a modern sprint kart application, with a medium to high horsepower class, it’s almost a necessity to have at least one chrome strut per side. Lower horsepower classes or harder tires can get away without additional struts, as the don’t generate the same torsional cornering forces needed to twist the kart, so the softer rear lets the inside rear wheel lift easier. The difference between one and two struts per side is super minimal, but more struts will increase the weight transfer from the seat a touch more and give a more direct connection to the outside rear corners of the kart. Most won’t notice the difference in feel between one or two struts per side. As a smaller driver, I run two per side to get every last bit of weight transfer I can from the seat since I don’t have a high center of gravity. Also, if you happen to break a strut or lose a bolt on one during a race, it’s good to have a spare still attached.