Plurals don't need an apostrophe
The first thing to remember is that a plural never takes an apostrophe. A simple plural just adds an 's'. There are some irregular plurals, of course, but they still don't need an apostrophe.
three dogs, 500g carrots, high winds, nine doctors, ninety-nine politicians, 34 MPs, 4,000 pages, 100,000 words, five children, 66 sheep, two chimneys, five stories
Pronouns don't need apstrophes
A pronoun is a word that is used instead of a proper name. They are quite happy standing in for their noun so they don't need an apostrophe hanging around.
his, hers, ours, yours, its, theirs, whose
Apostrophes just want to belong
When you do need an apostrophe is to show that something belongs to someone.
the lady's dogs, the GP's cats, the dog's collar
If the owner is plural, it takes an apostrophe after the plural 's' because the object is belonging to them, not because there's more than one of them.
the two ladies' dogs chased the three GPs' cats round the garden and landed in their friends' pond
Some nouns are already plural, but the same rule applies.
the children's house was next to the people's park
Too many ss
Where it does get tricky is in words that already end in 's' and grammarians come up with different solutions. So follow whichever one of these guidelines you prefer.
Option one
One-syllable words take an apostrophe and the s.
Les's clothes
Words with more than one syllable just take the apostrophe
Jesus' parables
Option two
If it sounds okay, do it
If it sounds like you are turning into Kaa, the snake, leave off the extra s