This and similar roots were used for Elvish words for “one” for most of Tolkien’s life, though generally in competition with √ER. The root first appeared as unglossed ᴱ√MĪ in the Qenya Lexicon of the 1910s with derivatives like ᴱQ. mir “one” and ᴱQ. minqe “eleven”; the parenthetical comment (mi’i) indicated it originally had some other now-lost consonant, though Tolkien marked this comment with a “?” (QL/61). It also had derivatives in the contemporaneous Gnomish Lexicon such as G. min “one single” (GL/57).
The Qenya Lexicon of the 1910s had a distinct root ᴱ√MINI from which “tower” words were derived, such as ᴱQ. mindon/G. mindon (QL/61; GL/57). In The Etymologies of the 1930s these roots were merged together into a single root ᴹ√MINI “stand alone, stick out”, whose derivatives included both ᴹQ. min/N. min “one” as well as ᴹQ. mindo and N. mindon/minnas “tower” (Ety/MIN).
In Tolkien’s later writings √MIN was mostly mentioned in the context of Elvish numbering systems, and given glosses like “one” or “first of a series” (VT42/24; VT47/16-17), but in notes written in 1967-69 Tolkien indicated it was still the basis for “tower” words, most likely from the sense “prominent” (VT42/24). In these later notes Tolkien was careful to distinguish √MIN “one” used in counting vs. √ER “one” used for individual things: √MIN was for the first of a series of things, whereas √ER was for a single thing that was unique, alone or in isolation. The distinct senses of these two roots dates back at least as far as The Etymologies of the 1930s (Ety/ER, MIN).