I think I heard somewhere a rule that says "numbers up to twelve should be spelled out, numbers above can be written numerically" but not only do I not remember if that was exactly how the rule went, I'm fairly sure the source wasn't really reliable.
So, can you tell me whether - in a common text, not legalese, not calculations, nothing unusual - I should use digits or letters? Is it true you should write "eleven" but "13" in a literary text?
Also, I'm pretty sure that, even if it's true, there must be exceptions. If I'm right, can you name them?