Q. Sentence: Only 1 in 66 households [has/have] received this letter. Is it has or have? I presume that because 1 in 66 is the lowest common denominator of a larger group it should be have.
A. If literally only one household received the letter, using the singular has would be the intuitive (and correct) choice. But normally this construction expresses a ratio with a plural numerator, as you suggest, so that “1 in 66” might actually stand for, e.g., “200 out of 13,200.” Perversely, the singular verb is still recommended by many, perhaps because the word one is the subject regardless of its implied meaning in such expressions.