"Ellos quieren las seis sillas."
Translation:They want the six chairs.
This should be less strict with the articles. "They want six chairs" must be accepted in this case.
It's an unusual sentence to be fair. Say you were selling a table and a set of six chairs but they only wanted to buy the chairs, you could say "They want the six chairs". Whether that's what's implied by the Spanish sentence, I don't know.
It could be considered correct in a rare situation for example you had previously discussed chair purchases and you explicitly state you want 'the six chairs' you previously discussed. Otherwise 'They want six chairs' should be accepted as correct in this case and almost every case.
In my head, I thought "las seis sillas" sounded weird, so just put "six chairs."
Might just be me, but this sentence doesn't sound natural in English or Spanish.