, or seven, or nine, or eleven; all of that is permissible, and the majority of scholars have adopted these forms because the reports confirming them are sound.
Had Witr been an obligatory act, its rakʿah count would have been fixed and well known, with no allowance for addition or subtraction—exactly like the five prescribed prayers.
As for Imām Abū Ḥanīfah, who considers Witr compulsory, he rules that it must be three rakʿahs: it may not be prayed as a single rakʿah nor as more than three, nor may a traveller perform Witr on his mount, since in his view it is a duty akin to the obligatory prayers.
Yet his earliest companions disagreed with him on the obligation of Witr, and only some later adherents of the Ḥanafī school accepted his view. The previously cited evidences—and others besides—support the position that Witr is not obligatory.