public static bool useLegacyProbeSampleCount { get; set; }

The fixed sample counts are: 64 direct samples, 2048 indirect samples and 2048 environment samples. Disabling this ties the Light Probe sample counts to the sample counts specified for lightmaps. When disabled a multiplier is provided in the lighting settings to be able to use more samples for Light Probes. This is because a Light Probe covers a larger area than a lightmap texel so generally needs more samples to converge.