The Mandaic alphabet is called the abagada, after the sounds of its opening letters, in the same way that alphabet is named after the first two letters of Greek. Naming a script by its beginning is common. Ending it on the same letter it began with is not.
The traditional count is twenty-four: the twenty-two inherited letters, then dushenna, then halqa written again. The last sign of the alphabet is the first sign of the alphabet, and the sequence closes as a circle rather than as a line. That is a fact about how the alphabet is recited and counted, and it is stated here without an interpretation attached to it.
The interpretation is where care is needed. Mandaeism holds its script to be sacred and its letters to carry creative power, and copying the scriptures is a priestly act. What that means in the tradition's own terms belongs to a priesthood entered by initiation, and it is not open material. The atlas can record that the community holds the letters in this way, because the community says so; it cannot set out the doctrine behind it, and it should not reconstruct one from the outside.
The distinction matters for this letter above all the others, because a closed circle invites a reader to supply a meaning. Most published speculation about what the repetition of halqa signifies comes from outside the community. The atlas records the count, names it as the tradition's own, and stops there.