I can only answer about humans. You ask what is needed to control a limb... I assume you are only talking about conscious motor functions.
- "Conscious" means that we must exclude everything related to the peripheral nerves that inervate the limbs, as well as reflexes (these are located in the spinal cord)
- "Motor" means that we must exclude everything related to the
"sensory" neurological processes
These exclusions are artificial, since the limb movements inherently integrate these neurological processes. For example, without the peripheral nerves, there would be no way to transmit the instructions from the brain into the muscles. Also, without sensory processes, movement would be more difficult... so, for instance, proprioception allows the brain to know where each part of the limb is located in space, and therefore, how the limb can move in order to achieve the desired effect.
However, in order to simplify my answer, and since this seems to be what you're asking, I'll restrict myself to said conscious motor functions.
Schwern is correct in answering that the brain simply doesn't function like that. It is not so much of a matter of percentage of brain, or of number of neurons... what is important is the number of neurological connections (synapses) involved in a function, and the complexity of those connections.
But, even so, I'll try to answer the question "What percentage of the brain does it take to control that limb?
Our brain is covered by a superficial layer, that is called the cerebral cortex. It is in this layer that our neurological connections are the most complex... and therefore, it is in this layer that the most superior brain functions are located.
So where in the cerebral cortex are the motor functions located?
If you watch a brain from above, you'll see a fissure that divides the brain in half - a left half and a right half. Those are the hemispheres. The left hemisphere of the brain controls the motor functions in the right half of the body and the right hemisphere controls the motor functions in the left half of the body.
If you watch the same brain from the sideways, you'll see another groove that divides the brain in two parts - an anterior part and a posterior part. This groove is called the central sulcus. The motor functions are located on the front part, the frontal lobe, right adjacent to that central sulcus... this is the motor cortex.
If you take this part of the brain and slice it on a right-left direction, you'll be able to see the cortex enveloping the brain. Now, if you superimpose on each part of the motor cortex the body parts that it controls, you'll be able to draw a "cortical homunculus".
As you can plainly see, the motor cortex that controls the hand is much greater than the motor cortex that controls the rest of the arm. Also, the motor cortex that controls the leg is much smaller than the motor cortex that controls the face. This is because the hand has much more nuanced and fine movements than the arm... and all our facial expressions need a much finer motor control than the simple movements needed to move a leg (which consist almost exclusively in moving it forward or backward). So, the amount of neural connections needed to control all the hand and face movements needs more brain space than the leg.
So, it is not a matter of how much limbs your being has... but of how fine the movements needed for said limbs are.
Please note that this is an oversimplified answer... To be more precise, I would need to detail a lot of other brain centers, namely responsible for motor cohordination. The motor cortex that I detailled above is only responsible for moving the limbs, not to cohordinate those movements with the other parts of the body. So, if you electrically stimulate the "leg part" of the motor cortex, your leg will jerk accordingly, but that movement may be extremely imprecise for any objective (vg: kicking a football).
PS: Here's another image of a cortical homunculus, ie, a representation of the body in which each bodypart has a size directly proportional to the respective amount of motor cortex.