A word glossed “countless” in Definitive Linguistic Notes (DLN) from 1959, a combination of negative prefix ú-, nod- “count”, and the suffix -ui “having quality” (PE17/144), so more literally “✱un-count-able”.
Sindarin
únodui
adjective. countless
únodui
adjective. countless, *uncountable
adj. countless. >> ú-, -ui