Use a Particle system.

Set it to [Hair mode](https://docs.blender.org/manual/en/dev/physics/particles/hair/index.html) and set the Render to Object.


Select the object you want to use as spikes as Dupli Object.
(The specified object will be duplicated in place of each particle)



[![enter image description here][1]][1]

Use [Weight Paint](https://docs.blender.org/manual/en/dev/sculpt_paint/painting/weight_paint/introduction.html?highlight=weight%20paint) to define the areas where you want the particles to appear.

Red where you want the most particles, blue where you want none.

[![enter image description here][2]][2]

Then set the [Vertex Groups](https://docs.blender.org/manual/en/dev/physics/particles/emitter/vertex_groups.html)
 to use the vertex group created through the weight paint.

[![enter image description here][3]][3]


  [1]: https://i.sstatic.net/gvLgG.png
  [2]: https://i.sstatic.net/oPjRa.png
  [3]: https://i.sstatic.net/ZIeTR.png