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).
The earliest Elvish words for “eight” were ᴱQ. {ungo >>} umna and G. {ung >>} uvin in the Gnomish Lexicon of the 1910s (GL/75), but in the Early Qenya Grammar of the 1920s it became ᴱQ. {telte >>} tolto (PE14/49). In The Etymologies of the 1930s Tolkien gave the root ᴹ√TOL-OTH/OT “eight” as the basis for ᴹQ. tolto and N. toloth of the same meaning (Ety/TOL¹-OTH/OT); in this document it was distinct from ᴹ√TOL which was the basis for “island” words.
In documents on Elvish numbers from the late 1960s, Tolkien vacillated between √TOLOTH (VT42/30 note #52), √TOLOT (VT42/24; VT47/31) and √TOLOD (VT47/11) for the form of this root, but in the more polished versions of these documents he seems to have settled on √TOLOD > Q. toldo, S. toloð (VT48/6). In this last iteration, Tolkien connected the root √TOLOD to the root √TOL “stick up” due to the prominence of the middle fingers (3 and 8) in counting (VT47/11); see the entry on √TOL for discussion.