- When there's a reason to stress the number, you add the accents. Otherwise, it's ruled by the context:
- If the sentence holds meaning both if it's the article and if it's the number, then again you add the accents for the number. (And you don't if it's the article.)
- If the sentence holds meaning for just one interpretation, than you don't add accents, unless for stress. This means that one is not always written with accents! E.g., I always wanted a puppy, and now I have one. "Ik wou altijd al een hondje en nu heb ik er een."
Regardless of the spelling, though: If it's the number, it's pronounced with an English "ay", where the article is pronounced with a schwa