The counting of the psalms as 150 did not become fixed until late in Hebrew tradition. The Hebrew manuscripts of the Bibliothèque Nationale de France display an amazing variety in this regard, their numberings ranging from 146-157. Moreover, in the case of numerous witnesses, the numbering sequences have been added secondarily, without attention to the Psalter's format, e.g., the 'Egyptian Hallel' (Psalms 113-118) appears in at least twelve different configurations. The hesitations of the manuscript tradition regarding the precise divisions of the units making up the Psalter reflect those of the rabbinic sources. [Adapted from published abstract--Christopher T. Begg].]
Auwers, J.-M. (2002). La numérotation des psaumes dans la tradition hébraïque. Une enquête dans le fonds hébreu de la Bibliothèque Nationale de France. Revue Biblique, 109, 343-370. https://hdl.handle.net/2078.5/155109 (Original work published 2002)