A singular noun takes apostrophe-s. A plural that already ends in s takes s-apostrophe. Apostrophes do not make ordinary plurals. The student's score. The students' scores. Two students, not student's for the plain plural. Apostrophes as a set are in apostrophes.
Ownership, not plurality
Ask whether something belongs to the noun. If yes, add the apostrophe. If you only need more than one of the thing, skip it. Pronoun possessives never take apostrophes: its, whose, your, their. The contraction pair is in its versus it's.
Who's versus whose is in who's versus whose. Punctuation overall is in ACT punctuation rules.
The bottom line
Ownership: apostrophe. Plain plural: none. Pronoun possessives: no apostrophe.
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