N4
も（数量）
も（すうりょう）
Emphasis, As ~ as, Not even
１２時間も仕事をしたから疲れた。
I worked as long as 12 hours, so I am tired.
Explanation
「も」 after an amount means the speaker thinks it is a lot, and is a bit surprised.
「10万円も払った」 (I paid as much as 100,000 yen). 「本を3冊も読んだ」 (I read as many as 3 books).
With a negative it means 'not even one': 「1人もいない」.
Mixed up with
「10万円も払った」: paid that much! 「10万円しか払わなかった」: paid only 100,000, which feels too little, and a negative must follow. 「10万円だけ払った」: paid just 100,000, with no feeling.
「も」 goes right after the number + counter; the noun keeps its particle: 「本を3冊読んだ」 → 「本を3冊も読んだ」.