"She does not have one enemy."
Translation:Níl namhaid amháin aici.
Cardinal numbers are used like adjectives but come before the noun, not after it, and adjectives usually do.
amháin isn't a cardinal number, it's just an adjective, but it's used in place of the cardinal number aon when used with a noun. "one enemy" can be translated as namháid, namháid amháin, or aon namháid amháin, but aon namháid actually means "any enemy".
As Sean52990 points out, numbers over 10 are a bit more complex.