You really couldn't tell unless you had context. Masculine nouns are weird like that, because the ones that don't end in "a" have no plural form.
As shivj80 says, masculine nouns that don't end is "a" can be either singular or plural. I always use the plural form. On Duolingo this seems to work.
This translation could be: Raj give me the mango & Raj gives me mangoes, BUT could it also be Raj give me THE mangoes?
is that valid even if they don't say एक आम? not trying to refute, just curious
एक is really unnecessary, unless you are particularly stressing that is was only one. I think the एक is used a lot in these lessons, as a translation, as there really is no "a" in Hindi.
Please anyone, correct me if I'm wrong.
It cannot be 'Raj give me the mangoes' because it would need the imperative form. In its current form it is not an order, just an observation.
'देता है' is the present tense. It would need to be in the imperative form to signify 'Raj, give me the mango' would be something like this, which we haven't come across yet.: "राज मुझे आम दे दो।" (informal) "राज मुझे आम दे दीजिए।" (formal)