Garçon is used for boy or kid, but it is unacceptable and disdainful to call a waiter "garçon"! This is like saying in English, "hey you","boy" or "honey" to a server!! In France it's why waiters sniff their Gallic noses, and won't wait on you! Try Monsieur or s'il vous plait!! Works much better!
This is where deduction comes in handy. If you're in the possessions section of DuoLingo, and you don't hear a possessive, then it's probably an error in hearing or in speaking. Either way, if you take the time to think about it, you can easily figure out that the word meant is "leur," rather than "le."
Unfortunately, I didn't take the time to think about it...
of course, I'm aware of conjugation, but I had something different in mind-perhaps my question wasn't clear enough. I was thinking about hearing a short answer to some question in conversation- just hearing "Leur garcon.", and not knowing if it's 1 or more boys. Anyways, it seems that you have implicitly answered to this-"leur garcon" and "leurs garcons" are indeed both pronounced exactly the same and we do need to look for additional things to be sure. Thanks for your numerous commentaries on many different issues raised here on Duolingo; they make it much more efficient tool for learning French (at least for beginners such as myself).
I think you have learnt lesson nb1: French marks of plural -s (and -x) are mute.
Now, lesson nb2: they are pronounced as liaisons with a Z sound if the next word starts with a vowel or a non aspirate H:
- leurs-Z-amis (their friends)
- de beaux-Z-hommes (beautiful men)