No, not always; a singular noun as well as a plural noun will show possession.
Singular: The teacher's desk had been repainted.
Plural: The teachers' desks had all been repainted.
The possessive form for the plural noun wives is wives'.
The possessive form for the plural noun cubs is cubs'.Example: The cubs' mother watched them closely.
The plural possessive noun of molecules is molecules' .
The plural form for the noun buzz is buzzes; the plural possessive form is buzzes'.
The plural possessive noun for "vowel" is "vowels'".
The word sisters is the plural form; the plural possessive is sisters'.
The noun "branch" is a regular plural noun, a noun that forms its plural by adding "s" or "es". The plural form of branch is "branches".The plural possessive form is branches', a regular plural possessive.
The plural form for the noun tree is trees.The plural possessive form is trees'.
The possessive form of the plural noun plays is plays'.example: His plays' endings are always so sad.
pLURAL ; orbits Possessive plural ; Orbits' (NB note the position of the apostrophe).
The plural possessive noun is racers'.
The word Kiowas is the plural form for the proper noun Kiowa, a nation of Native American people. Proper nouns are always capitalized.The possessive form for the plural noun Kiowas is Kiowas'.