The Mandaic alphabet can be counted in more than one way, and the numbers that result are all correct answers to different questions.
The tradition counts twenty-four. That is the abagada: the twenty-two inherited letters, then dushenna, then halqa written a second time to close the sequence where it began. It is the count a recitation reaches and the one the alphabet's own name belongs to.
The modern character standard encodes twenty-five, because it must also carry kad, the ligature of ak and dushenna, which the traditional reckoning does not name, and ain, a late borrowing from Arabic. A standard has to serve editions of manuscripts as well as the count a community keeps, and a sign that appears in manuscripts has to be encodable whether or not it is a letter.
Counting only the inherited signs gives twenty-two, which is the number that matches Hebrew, Syriac and the Arabic abjad reckoning, and it is the right number for any comparison between the alphabets.
None of these is the true count and it is worth saying so plainly, because a reference work that prints one number without saying which question it answers leaves the next reader to discover the discrepancy and assume an error. This atlas holds twenty-five rows, one per encoded sign, and records on each of them where it stands in the tradition's own reckoning.