530 is like the chain size.. (eg #35 chain goes on a #35 sprocket with x amount of teeth) So a 530 sprocket would use 530 chain and have a certain number of teeth on it
530 sprocket is pretty beefy too.
Most of the time these sprockets/reverse gears are for the 150cc or 250cc engines that are geared differently than a go kart would be with an industrial type motor. You would have to have a good understanding of gear ratios if you were (for example) trying to put a reverse gearbox on a go kart with some 5-7 hp briggs/honda/predator industrial engine on it.