* If "in" is used as a preposition, it remains in in the plural.
Example: *Les livres sont in the box.* (The books are in the box.)
* If "in" is part of a compound word, the pluralization rules of that specific word apply.
Example: *Les inch are equal to 2.54 centimeter. * (The inches are equal to 2.54 centimeters.)
Let me know if you have a specific example in mind, and I'll be happy to help you with the plural form!