How many letters does Glagolitic have? The honest answer is that it depends what you are counting, and pe is one of the letters that makes the question unavoidable.
The oldest Slavonic discussion of the alphabet, the treatise on the letters by the monk known as Chernorizets Hrabar, gives a figure of thirty eight. The surviving manuscripts do not agree with that figure or with each other. The atlas lists forty four, following the arrangement of the Unicode Standard, which encodes the signs attested across the whole tradition, including regional forms, positional variants and letters like this one that are known chiefly from lists.
Each of those counts is answering a different question. Hrabar is reporting what he understood the alphabet to be. A manuscript shows what one scribe in one place actually used. Unicode is providing addresses for everything anyone might need to typeset. None of them is a reconstruction of what Constantine made, because the evidence for that does not survive.
A reader coming to the script for the first time is better served by knowing that than by being given a number. The count varies because the object varies, and a sign attested only in an alphabet list sits exactly on the boundary between a letter of the alphabet and a mark that a scribe once drew.