This tool is a special one in Sketch. It’s not in the toolbar by default, but you can add it by right-clicking the toolbar and choosing Customize. It’s also available in the menu under Layer › Paths › Rotate Copies.
This tool takes one layer and rotate copies of it around a certain point. Take the example of a flower; create just one leaf, then rotate a dozen copies around the center and you’ll have your flower.
Select a layer, activate the Rotate Copies tool, enter the number of copies and close the dialog. Then position the center-dot where you want it and click outside or deactivate the tool to exit it.
All shapes will be added as subpaths to the original shape. If you’d rather have them as completely separate layers, go to Layer › Paths › Split in the menu to turn them into separate layers.