More months in a year than days in a week
Yes, this sentence sounds right to me. You've also constructed it correctly!
According to Merriam Webster, "than" is used to express the second member/second part of an unequal comparison.
In your sentence, I infer that there are 12 months in a year and 7 days in a week. Both numbers are not equal, so you're evidently on the right track!