"In" is a preposition, and prepositions don't change based on the number of things they refer to.
If you are looking for a plural form of a word that uses "in" like "inside" or "within," those words already have plural forms. For example, "dentro" (inside) remains the same in the plural.
Example:
* English: The books are inside the box.
* Spanish: Los libros están dentro de la caja.