I have a number of intelligent alien species with a high number of limbs (from 6 up to 18) some with radial symmetry and some bilateral, in most cases the creatures can crawl using all limbs but when doing task the hind legs (3 to 6 depending on limb number) become the legs supporting their weight as the raised up front of the body uses the rest of the limbs as arms. I also have other species with very high numbers of simple appendages, similar to echinoderms.
I have some species which I have elements of their physiology but I have not decided on the limb number (although I do prefer more than 4 with 2 as arms). I believe there is a point where more is not better, as they will involve more brain power, blood/nutrient supply and sensory methods, and there comes a point where less limbs can reach all the places and carry out the tasks that more can.
These alien designs will be for my technologically advanced species, they will be spacefaring and will need to complete tasks and progress in a way that is similar to humanities path to becoming technologically advanced.
Is there theoretically a most efficient number of arm-like appendages, where anything more requires too much additional support systems that it would in efficient for a creature to evolve and supply necessary requirements for survival?