The Quenya number “eight” derived from the root √TOLOD, probably from primitive ✱✶tolodō, with the middle vowel lost due to the Quenya syncope.
Conceptual Development: The earliest attested Qenya word for “eight” was ᴱQ. umna in the Gnomish Lexicon from the 1910s (GL/75), but when Tolkien composed the number lists in the Early Qenya Grammar from the 1920s, it was revised to ᴱQ. tolto (PE14/49, 82). In The Etymologies from the 1930s it remained ᴹQ. tolto from the root ᴹ√TOLOT (Ety/TOL¹-OTH/OT).
When Tolkien revisited the Elvish number system in the 1960s, he first used tolto (VT47/32), but he later changed the t to a d in both the Quenya form and the root (VT48/6).
Neo-Quenya: I personally prefer toldo as the Quenya word for “eight”, but some Neo-Quenya writers use the older (and perhaps better known) tolto. It seems Tolkien had considerable trouble deciding on the primitive root for “eight”, so any of these forms could be valid (VT47/31).
toldo, see tolto