The SOLIDWORKS 2018 Circular Sketch Pattern command is available for both 2D sketch entities and 3D features. There used to be a limit of 1000 instances for patterned instances in SOLIDWORKS 2017 and older versions.
But in SOLIDWORKS 2018 there is almost no limit for instances of circular sketch patterns. In the following image, it is shown that the maximum number of instances could be 1,000,000,000 instances. Therefore logically, no limits exists for the number of instances for circular sketch patterns in SOLIDWORKS 2018.
Note: that for circular feature patterns there was 999,999,999 instances limit which has not been changed in SOLIDWORKS 2018. Therefore, the instances of circular feature patterns seem to be logically limitless.