I know this is an old post and I hope you figured it out, I don't know for certain, but I currently suspect 'leatsa' (with you) in one of its forms (is leatsa [is yours], is leatsa é/í [it's his/hers/its] etc) is used for the singular of 'your/s', and bhur for the plural. That's my best guess so far.
If you meant 'how are you supposed to guess if Duolingo want you answer in the singular or plural', then I don't know. I just go by the context of the preceding questions. If there's a run of questions about plurals, then my money is on them being after a plural in the next question. Doesn't always pan out like that though.
English doesn't differentiate between singular and plural "your". For English->Irish exercises, Duolingo has to try and take account of that ambiguity in English, but accepting both singular and plural versions of the answer.
If you are talking to two or more people, you would say bhur n-arán - "your bread".
If you are talking to just one person, you would say d'arán - "your bread".