Most of the time, either the number of the noun is clear from context or you are completely fine not knowing what it is.
“They are waiting at the door for you.” Is there one of them? Is there more? Who cares, the sentence covers both!
Granted, Japanese has syntactic gadgets that enable you to add that information in a natural way in cases where you actually do care, but it’s still surprising at first how much communication works without obsessively marking every noun for number. How do Japanese, Chinese, and Korean speakers communicate without grammatical number? The same way Russian speakers communicate without grammatical definiteness, or English speakers communicate without grammatical perfectiveness, or all of them communicate without grammatical clusivity, etc.: either they mark it explicitly or—in most cases—they can perfectly well do without. It’s technically ambiguous, but only in the way every other word that isn’t marked for every grammatical category found across human languages (i.e. every single word) is ambiguous.
(In case you were wondering why speakers of Slavic languages sometimes tend to overuse “some” and “this” in English, that’s why: those are the way their native languages, lacking obligatory articles, mark nouns for definiteness when it actually matters; the overuse is what happens when you teach articles as “these weird formal words you have to use according to these weird formal rules” without telling your students that they actually bear meaning.)
"They are at the door right now, please open it asap [and let a singular individual in]". If that bracketed portion is the intent, requires context. It implies a specific individual who you could identify by name or by a gendered pronoun if known (and unique in context) or description. Either way, you need to let the person you're speaking to know which "they" of the potentially many individuals at the door you want to permit in.
This context may have already been supplied. For instance, on visiting an office for an interview a name will already be supplied and the person (security, admin) doing the opening may already know the identity and that it is a singular person who is expected. In which case, "they", because of prior context, is entirely unambiguous again.