Tuomo Pekkanen's grammar (§92.1) explains how to express the number of something that is expressed by a plural-only word. Numbers greater than one are expressed with bini, trini etc. but a single one is not singuli. The cardinal number unus is used instead.
For example: unae scopae, binae scopae, trinae scopae… (The actual example given in the book is una castra.)
This strikes me as odd. But then again, Latin grammar is occasionally odd. I would like to have verification or falsification for this rule. Are there classical use examples like "one broom" (unae/singulae scopae)? What do other modern grammarians say on the topic? Is unae correct and singulae incorrect?
Perhaps the neuter plural una (as used in the book's sole example) looks a little less suspicious than unae, but I still find it weird.