Native French speaker here. Maybe it's confusing to discuss this in terms of syllables unless you are a linguist. If you just want to learn how to say the word, what matters is how people actually say the word. Specifically, you pronounce "rien" in one quick shot. (So it's definitely closer to one syllable if we care about this. But if I am to say the word very slowly to teach somebody how it sounds, it might progressively become closer to two syllables. Languages are complicated.)
>But if I am to say the word very slowly to teach somebody how it sounds, it might progressively become closer to two syllables. Languages are complicated.
I'm sure that's true, but I don't think it gives rise to any real doubt as to the syllable count. For example, you could stretch out the English word "near" into two syllables in very slow and deliberate speech, but it's definitely a monosyllabic word in most (possibly all?) varieties of English.
For "rien" in particular, I'd say speakers can do that diaeresis as a sort of emphasis. For instance "Tu n'as rien fait! Rien! ("you did nothing! Nothing!"). The first "rien" can be said as a single-syllable world, while the second one can be pronounced as a two-syllables.
It also may be pronounced slightly differently in various places in France (regional accents).
