Having more limbs is less efficient biomechanically - the creature would need a larger heart to pump blood through al organs, that requires more energy, so such creature would need more food.
Now, it is known that creatures can lose limbs through evolution, but never gain them, so it is likely that multi-legged species would gradually lose extra limbs over time.
Now, it is known that creatures can lose limbs through evolution, but never gain them, so it is likely that multi-legged species would gradually lose extra limbs over time.