√LEPEN was the most common root for “five” in Tolkien’s writings, but he explored a variety of other options. Its earliest iteration appeared in the Qenya and Gnomish lexicons as ᴱ√LEH (QL/52) or ᴱ√LEF “half” (GL/53), so I think the actual early form was ✱ᴱ√LEǶE [lexʷe]. At this early stage it had derivatives with the meanings “five”, “ten”, and “half”, but in later writings “ten” became ᴹ√KAYAN >> √KWAY(AM) and “half” became √PER.
In The Etymologies of the 1930s the root appeared as ᴹ√LEPEN “five” with variant ᴹ√LEPEK, but ᴹ√LEPEK had no derivatives (Ety/LEP). √LEPEN appeared again in a list of numbers from the late 1950s or early 1960s beside a variant √LENEP; again the variant had no clear derivatives (PE17/95). √LEPEN reappeared in numeric discussions from the late 1960s (VT42/24; VT47/10). In these late discussions Tolkien said that “five” most likely originally from ✱lepem as an ancient plural of √LEP, but it seems this became √LEPEN already in Common Eldarin (CE), given that the Sindarin word for “five” remained S. leben; Tolkien gave varying explanations for this CE sound change, either as dissimilation from p (VT47/26 note 2) or with final -m > -n being the regular phonetic development (VT47/24).
This root was connected to Elvish words for “finger” for most of Tolkien’s life. It first appeared as an unglossed root ᴱ√LEPE in the Qenya Lexicon of the 1910s with derivatives like ᴱQ. let (lept-) “finger” and ᴱQ. lempe “crook, hook” (QL/53). There were also derivatives in the contemporaneous Gnomish Lexicon such as G. leptha “finger” and G. lempa- “beckon, crook the finger” (GL/53). The root ᴹ√LEP appeared in The Etymologies of the 1930s with extensions ᴹ√LEPET “finger” and ᴹ√LEPEN “five” and various derivatives of similar meanings (Ety/LEP).
In Tolkien’s later writings, √LEPEN “five” continued to appear regularly, along with the base root √LEP that was glossed either “finger” (VT42/24) or “pick up (with fingers)” (VT47/10, 24, 27). Despite the stability of the root, the Elvish words for “finger” themselves when through many revisions; see Q. leper and S. leber for discussion.