6 tracks in a triangle shape with a hexarm allows single atom handoffs between all grippers, which keeps everything relatively easy to handle. Anime, calc, 1 bonder, both inputs, and the output all need full access unless there's a debonder, so there are 10 hexes of required access. One single arm has #track access points, and two single arms have at most 2*(#track-1) access points, so single arms cannot be cheaper than a hexarm. A hexarm with 5 track might be possible, but it is much harder since handoffs become very restricted.