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.
They are aquatic species. When you remove the necessity to support weight above the hard surface, your options are much more varied. Even jellyfishes or amoebas somehow manage to get by.
Early creatures had mady different body plans. We have 4 (all vertebrates), 6 (insects), 8 (arachnids) and multilegged creatures (millipedes), as well as legless creatures like snakes (who had 4 legs before they lost them).
It seems that vertebrates have certain period when this layout was 'cast in stone' [1]
Reading your comment made me wonder why we have even numbers of limbs for different body plans. I realized it's due to bilateral symmetry for most organisms must some how be advantageous to asymmetric body plans, but I'm unsure how symmetry is advantageous. What are the evolutionary pressures selecting against asymmetric body plans?
I'm not really a biologist, but I think this has something to do with the balance: when you move in some direction, it would be extremely disadvantageous to have one side significantly heavier than another.
Take two runners, give one a 15 kg weight in one hand, and give another 15 kg weights in both hands (30 kg total). I think the runner with weights in both hands would run faster than the one with one weight.
Thus, moving species evolve to be bilaterally symmetric.
If the organism has no need to move in certain direction, it doesn't really have bilateral symmetry, like trees or bushes (though big trees generally tend to have their center of mass close to their vertical axis).
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.