Create a cylinder with 24 vertices for a 12 spike object. Set the Cap Fill Type to nothing.
Tab into edit mode and select the bottom ring of vertices. From the Select menu select Checker Deselect. Now use g to grab the vertices and z to constrain the movement to the Z axis. Drag down to set the length of the spikes. In the following image I have hidden the top ring of vertices with h
Then extrude the top ring twice, once to create the ridge and the second time to create the top. Close this with alt+m and Merge to Center.
Now Tab out of edit mode. Set Smooth and add the Subdivision Surface modifier and ensure the Subdivision for both Preview and Render are 1.