I know you didn't ask, but it's a bit more complicated than that. They usually come in groups of 3, and if you get all three, then the last hit on the third one will give out 500 points.
If there are more than three in a set, then the last hit on each one after that will also give 500 points. Sometimes this is a bit bugged, for instance, in Casino Night Zone 2, if you skip the set of six near the start of the stage, and then a bit after that get all three from the lower of the two stacked sets, you can then go back to the six from earlier and find that they all give the 500 point bonus now -- it just happens that those six and the three from the later set belong to the same group internally.