You can choose the number. It is controlled indirectly by the aa setting.
Each ray intersection sends a ray to each light to see if it's visible. If you has 1 million pixels and 16 samples per pixel they you will get at most 16 million smaples per light assuming no reflections or transparency. The background doesn't need to sample the light so it could be fewer.
BTW if what you want is to take more samples per light for each ray intersection instead of just one, it's better, say, double the number of samples per pixel at the start than to quadruple the ligt samples but not increase the samples per pixel.